The party (4 lvl 14s) are heading into an ancient and long abandoned temple to retrieve an artifact stored there. My current thinking is the temple is of sehanine, although it is pretty much unimportant and can be any good/neutral deity. 

So I need to fill the underground of this temple with a variety of traps, puzzles and other hazards. The temple itself is located on a tropical island within a jungle if that helps for thematic ideas, I'm currently thinking of using some acid pools etc as environmental hazards. Any ideas would be appreciated.

Another problem I'm starting to run into is that at this level I am either needing to use 'boss level' monsters are large numbers in order to pose any sort of challenge to the group.

Thanks in advance for any help or ideas!

I like your idea of acid pools and including acid darts (think Indiana Jones and the Lost Ark). I try and find my puzzles from other games. I play one called flow free that is a puzzle game where you have to connect differently colored pairs of dots. The lines connecting the dots can not cross each other and the entire play area needs to be filled. It is a free game on android and iPhone and may provide some ideas. I have also made a puzzle where players need to hit targets in a specific sequence based on the output of a mathematical equation (simple algebra, nothing crazy). I've also taken puzzles from other games/Movies (Witcher, Harry Potter, Labrynith, etc). 

Hope this helps a little.

Bugs, snakes, mold, traps of various kinds, something that has decided to make its home here in the temple.  It is cool and dry, and it might be much worse out in the jungle.  An ancient magic mouth is always great even if it isn't a threat.

One thing to consider is whether you have capacity here for environmental storytelling.

Why was the artefact stored in the temple, and what are the traps there to do?

Is the artefact meant to be locked away forever, and the traps are meant to frighten off anyone who would try and get it and create a superstition which prevents anyone from going into the temple, like in Indiana Jones & the Last Crusade, with "The Breath of God"?

Is the artefact meant to be protected only from certain types of people, and thus the traps are meant to remove the greedy or the evil but allow passage for the generous or good?

Are the traps meant to be in place for the original owners of the temple to walk through unharmed, knowing the way through the traps, whilst anyone else who tries dies, "proving" the holiness of the priests?

or are the traps meant to prevent anyone getting out, as the temple houses some dark magic which feeds on souls?

or, are they just meant to kill anyone who tries to get in, so they cannot get the artefact, because it cannot be destroyed and just walls will not keep people out for long?

Hopefully this will help you to theme any traps or challenges within the temple to match the nature of the temple & artefact!

I like traps that can be used against or by opponents.  Traps, for combat scenes, maybe hidden but in a predictable arrangement.  That shove action/thorn whip can be really great when there's a spiked pit en route.  Oh, oh, we're running, we're running - opps, did you just hit that tripwire that we jumped over? 

Work some logic between the items.  Some bubbling corrosive hazards have developed.  The air is hard to breathe.  The bridge is weakened.

Sometimes it's ok for players to do really well - if they know that playing badly would have really challenged them.

Quote from ThorukDuckSlayer >>

One thing to consider is whether you have capacity here for environmental storytelling.

Why was the artefact stored in the temple, and what are the traps there to do?

 100% this. To borrow a video game example, the buildings in Fallout 3 aren't there solely for players to loot; a school existed to educate students, not for raiders to loot a century later. A metro station is to aid transport, not to be the home of ghouls. From a game design point of view, it makes sense that these are 'dungeons' in a post-apocalyptic/'modern' setting. But that can still apply to this temple. The earliest I can trace back the temple raiding idea is to films like The Mummy or pulp horror novels that followed later, where disturbing the rest of the dead unleashes a curse. The Mummy of course being based on the Ancient Egyptian pharoahs, which were buried with their valuable belongings in gargantuan tombs to celebrate them.

So, as our resident slayer of ducks asks, what reason would an artefact be hidden away? It's one thing to disturb the rest of the dead, but why keep an artefact hidden away that nobody can see? I personally like their idea that this is the best way to contain the artefact, and that to let it out would do more harm than whatever traps may kill intruders. The traps are there for the raiders' own safety, or that of the island.

As for my own ideas for traps and cumbersome circumstances, here's some, in addition to the great ideas others have offered up:

• Stagnant water. Not necessarily a trap per se, but if the characters get parched and need a drink, they won't get much benefit from drinking water that has no running source (or it may have a long-dead creature in there). It could be a pool that calls for a poison or disease saving throw, and will not remove a point of exhaustion even if successfully saved against.
• Temperature. How well has this temple retained heat? The deeper they go, the colder it may get and might have to resort to all manner of things to keep warm. Again, their own mortality is the trap here.
• Magical mazes. Time is the danger here. Unless they can navigate the correct corridors and doors, they'll be stuck in a maze and may find themselves running out of supplies.Its DC is "Fighting Fantasy BS maze."
• Difficult terrain. The players may have to wade through waist-high water, only to be ambushed by annoying airborne creatures, or step on some sort of sticky creature that will grapple them. Dust might make it hard to see or breathe. And of course, illusions showing things harder to pass than they really are (see one of the earlier tricks in the Tomb of Horrors with the painted door, which is non-magical. That's just about the only inspiration I recommend you take from that dungeon.)

Dungeon Hazards and Traps can be combined with each other or with the denizens of your temple to increase difficulty and hazardous effect. There is some additional input on traps over in XGtE.

Your 1st paragraph notes that the temple is abandoned, but you don't mention if anyone/anything has moved in and inhabited the temple. Acid pools might work well if there were a creature that were acid-themed that inhabited the place.  Oozes, jellies and puddings are fun ideas for areas that don't get much traffic. Areas of poison gas might exist, or areas that were sunken and submerged. Rooms might be designed that would seal off and start to fill with gas, water or sand, forcing the PCs to solve a riddle or light a series of runes to stop the inflow of materiel, and a different series of runes to open the door forward.

Challenging the group can be accomplished by inducing waves of enemies, setting up the infamous "teleported back to the beginning of the dungeon without any of their clothing or gear" or by setting up areas that split the party and challenges each half, or individual, separately. A portcullis, portal or sliding stone door can all separate the party.

Magic use could be dangerous, depending how far underground the party is. Disturbing the structure too much could have unintended consequences for our intrepid heroes. <Rumble, rumble, collapse>

Rot Grubs can be deadly to any level of party, so long as enough are dumped on a creature to cause a hit. Fetid and stagnant water in the jungle surrounding the temple might cause diseases. Levels of exhaustion might be had if the temperature is very high in this area of your world. Getting to the structure might be a struggle all unto its own, especially if the entrance is thoroughly concealed to prevent looters from gaining access.