Does anyone have tips, some tested house rules or homebrews for running gargantuan creatures as more than just hard hitting bags of hit dice?

I feel that fighting giant, ancient dragon or tarrasque should feel more like shadow of colossus than just whacking away at bigger marker at the battle map. Like the creature could be a map on itself. I'm thinking of giving such creature large defensive boost, maybe damage reduction or threshold, or just higher AC, if characters can't reach some of their weak spots (softer scales at belly, a whole in giants armour, maybe going for eyes). To balance the hardness of getting to these weak spots, some of them might disable some of the creatures abilities (damage wings -> no more flying) and maybe deal extra damage.

Could also use complex trap style mechanics to move the creature in smaller increments (say, a leg every 5 initiative score) and to telegraph its larger attacks (Dragon's maw starts to glow and it inhales strongly).

Any thoughts? How would you track characters positions while climbing on a moving monster?

I think that is the idea behind legendary and lair actions.

If your objection is that large monsters are starting to feel like "bags of hit dice" - giving them more hit points, or making them harder to hit doesn't change this.

One thing that springs to mind with gargantuan creatures is they have the ability to re-write the map, simply by their size and actions.

If you're creating detailed terrain, and using it effectively, then creatures are less a "big bag of hit dice". Fighting a Red Dragon out on grasslands will be different than in a cave complex with pillars and stalagmites which the party can use for cover, but which the dragon can sheer off with her lashing tail. 

Additionally - large creatures can have minions. Cave complexes are likely to have entire ecologies of creatures - perhaps there are scavenger creatures living on the outskirts of the Dragon's lair. Perhaps a Dragon has acolytes and worshipers. 

In short, I think you can make the combat more interesting by making the situation and the environment more interesting, making the environment an element of the combat all its own, and allow the creature(s) unique tactical choices based on the situation and environment.

I think this is kind of what you're suggesting already, you're just collapsing it all into the creature itself :)

Yeah, lair and legendary actions likely take care of what I was going for with the complex trap mechanic.

My objection was mostly that hitting creatures toes with weapons it could use as toothpicks should not be the primary strategy when facing such creatures. I want to give the players the feeling that they are like mosquito buzzing around the dragon: annoying but not worthy of full attention. The players would want to avoid drawing too much attention while still having to stop the creature in some way. The idea of these extra defences was that when player character hits it, there is a reason to tell that their weapon bounces off and the creature doesn't seem to notice it. A clear sign that there must be a better way than to sip away scales one by one. The idea was that once the players find the weak spots and get to them, they should be able to make quick work of the creature or at least harm it enough to drive it away.

Interesting environment is always important for combat, and it is a good point that gargantuan creature will likely turn map that would be quite static in fight between medium creatures into dynamic one.

Yep - that's 5e taking cinematic and dramatic effects over super-hard-core realism. It's the game where Rouges with daggers, and Monks with punches can take out a Dragon.

What you are suggesting is kind of an Invulnerability/Vulnerability mechanic, I think. The party can't affect the creature very much until they find a vulnerability, at which point they can exploit it to great effect.

I would - however - caution you against structuring an encounter around the Players making a particular logical/tactical connection, or the Characters making a particular skill roll. "A clear sign that .... " often doesn't work out. What's clear to the DM can often totally escape the Players.

Know how you would want to handle it if the Party doesn't make the connection, and decides to  "[chip] away scales one by one".

For some reason, climbing on a back of a dragon to stab it or jumping of a tower to punch a giant in face, seems more cinematic than swinging at their legs. Any other ideas about how to promote this kind of action in combat.

But yeah, can't have a challenge that has only a single solution. Because of the last part, full invulnerability is out of the options, even though I would describe it to my players as such. I might even have the first encounter with a such creature in a way that if the players don't do something effective, the creature would just leave after couple of rounds, having fulfilled one of its own objectives. Then I could have a NPC give tips about the vulnerabilities before the next encounter.

If the creature is large enough to be the map, you can have its parasites defend it. You could also have the characters create an opening in the scales (or whatever). Because the creature is in motion, the part they are attacking moves about the map while others have a chance to crush them.

After getting through the hide, have the party navigate through tissue or blood vessels to seek out vulnerable organs guarded by “while blood cells”.

Quote from Xywzel >>

For some reason, climbing on a back of a dragon to stab it or jumping of a tower to punch a giant in face, seems more cinematic than swinging at their legs. Any other ideas about how to promote this kind of action in combat.

You might not need to invoke combat mechanics for this, at all, if cinematics are your only goal.

Reward players for large showy combat maneuvers and generally "being cool" by awarding inspiration. But the combat mechanics might not need to change - they might be just "skinning their attacks". My players know that if a combat goes particularly dramatically, I'll award a point of Inspiration, and then I allow the party to select their own most-valuable-player by secret vote to me ( where you're not allowed to vote for yourself ), and the player with the most votes receives inspiration. I don't typically dole this out often - just a reward for major battles handled really well.  

You can also encourage cinematic attacks, by presenting cinematic attacks from your creatures as well - again, doesn't have to be mechanically different, just described differently and dramatically. It also gives the players something to work off of, matching their own dramatic combat moves to the creatures'

I tend to approach this kind of thing by playing the dragon (or whatever) to the hilt. The dragon is probably going to control the circumstances of the battle, and its probably not going to stand still and be whacked on by a party of "puny mortals" with "toothpicks." It's going to do all it can to preserve itself and its hoard, even if that means escaping to fight--and revenge!--another day. It will use its breath weapon,  legendary and lair actions, and intelligence to its best advantage or take to the sky if necessary. A character that climbs onto a dragon I'm running will at the very least get the ride of his or her life. (What's the DC for hanging on to a dragon doing barrel rolls and zero g maneuvers? How high into the air can a cloud giant toss a medium-sized paladin in plate armor?) The point is, I don't see these big monsters as bags of hit points but as rare opportunities to play a really fun NPC (for the DM) and a really challenging foe for the players who, unless their characters are really high level, should consider roleplaying a negotiation or subterfuge rather than trying to fight it out toe to toe.

Quote from Vedexent >>

 

One thing that springs to mind with gargantuan creatures is they have the ability to re-write the map, simply by their size and actions.

 

This is an excellent idea. Fights with big creatures in movies and tv shows always include walls being smashed, pillars being broken, and so on.

I was in a party where we fought a giant dragon turtle from a boat with cannons. We had to roll two dice, each representing our hands to try and aim the cannons perfectly and the rolls dictated how much of a dead on shot we got. 

Of course me being a Monk I eventually dove in and much to the DM's chagrin stunning striked the thing so we could light it up but it definitely gave the encounter a large, and abnormal feel. 

It is really whatever  you want to make of it, I have had players that are on top of giant creatures make dex checks to see if they are steady enough to roll a regular attack or if their footing is uneasy make them attack with disadvantage to translate the difficulty of swinging while being jostled all over the place. The sky is the limit, but be warned there are alot of DMs who will nay say this sort of thing because of 'rules and reasons' But if you want them on the back of an insanely big dragon ala fighting Death Wing on WoW then you go for it, you know the mechanics and the rules as both written and intended, have fun with it and embrace your imagination.

If you can make it make some sort of sense to the players and they are having fun, do you. 

Instead of focusing on your tiny characters maybe have them obtain siege machines or get a flying ship or like an attack blimp that give them weapons or traps to deal with giant monsters. Your description of the fight reminds me of monster hunter or horizon last dawn where you need to target very obvious weak points.