Have your players ever found a bag of holding?  Were there contents already inside?  Could the players just look inside and see the contents?  Were there any traps inside when they reached in?

I'm adding a bag to loot in the next adventure location, and want to make it fun.  Please give me some ideas on how to make it not just a bag.

Yes, I've had players find a Bag of Holding... I don't remember exactly what was in it, but it belonged to a specific NPC they were fighting, and had a number of useful, but not rare items that the NPC just happened to be carrying... I think I just put the contents of an Explorer's Pack in there along with a lot of rations. But depending on where they find the bag could determine a lot about its contents.

The bag states the inside has specific dimensions, so I think it's reasonable to be able to look inside and see what's in there, but I've also seen some DM's play it as if there's basically a black void inside the bag and you have to know what you're reaching in for ahead of time in order to find it, but that's homebrew as the item description doesn't really mention anything about that. However, no matter what you can always turn a bag of holding inside-out and just dump out all the contents on the floor.

Here's what I recommend... have it be a necromancer's old bag of holding. A common tactic for Necromancer players is to get a bag of holding and keep a bunch of animated skeletons in there, so in emergencies they can just turn it insideout and suddenly they've got a gang of skeletons as backup. So either the necromancer lost it at some point, or maybe they got killed before they could pull out their skeletons... maybe they didn't keep track of just how many skeletons they had stored in there and forgot to reassert control over all of them, and one day they reached into their bag and got their hand chopped off by a skeleton they lost control over and bled out.

We had a bag of holding at the very start of the campaign, a family heirloom. It’s was more than 2 years (real life years, not in game) before we accidentally turned it inside out and found a very important item inside. I’m still impressed by how our DM kept the secret for so long. 

I have a few suggestions for encounters from a bag of holding

• Sometimes adventurers like to trap creatures in bags of holding. It could be pretty funny to have an angry redcap pop out when it's opened yelling at the players.
• There is also the classic of mysterious tentacles from no where which would just be a number of  giant octopus described as individual grabbing tentacles. It can also be fun to try and pull a player in to the bag
• An explosion of elements and elementals coming out is also an option and players don't often fight those. It could be mephits or one of the higher level ones. You have something dramatic like a  pillar of fire shoot out and then out comes a monster
• The bag of holding could be a living demiplane that pulls the players in for a fight
• the loot inside is a  mimic
• It's an alchemist bag and the chemicals have spilled. Inside is a mystery liquid with strange effects or that possibly releases a toxic gas. The liquid could cause a wild magic surge when it's poured out or when it comes in contact with a creature. The mystery potion could also make an ooze. 

There’s always the bagman. It’s from ravenloft

The bagman was an adventurer who tried to escape a battle by hiding in a bag of holding. He got lost in a universe of connecting extradimensional storage spaces and got twisted into a monster. Every night he emerges from a random bag of holding and if he doesn't find his party, he grabs a random person and drags the back with him into the bag. He leaves behind a trinket from his kingdom of lost junk. They say if you speak too loudly over an open bag of holding or say “follow my voice” three times into one, he will come for you. 

I had the party find a bag of holding, and I made a random table of stuff. They trapped an important enemy in the bag, but they didn't realise he could teleport (by flooding wherever he is in saltwater, which drains away to reveal he is gone). They trapped him, the bag swelled up and started leaking water, and when they turned it inside out, they found a whole bunch of junk - but he was gone, and so was the Hammer of Thunderbolts they had stored in it!

I play that the space inside appears as a void, and you have to know what you want to get it out. That way, it can be a surprise to find stuff already there!