@warzan: for monsters, how about going classic with a Beholder?

For encounters –

In Advanced Heroquest there was one Dungeon which had a room with one entrance and a switch and when the switch was hit the room rotated revealing a hidden room. Iirc the rotation was on a timer and after so many turns the room rotated again (90degrees each time) to reveal another room (and blocking off the previous hidden room) and it stopped once it’s done a complete circuit. I think rotation could also be sped up by hitting the switch again, but the switch could only be hit so many times before breaking which meant that heroes could end up trapped in one of the hidden rooms if not careful.

Another idea for moving rooms is to have a switch at the end of a corridor which when hit moves another part of the dungeon, opening up new areas and closing off old ones. You could have it set up such that the switch is one of the areas blocked off so that the players have to split the party, some going after the switch, and the rest going into the new area to find a second switch so that the first half can follow.

Talking of Elementals, how about a dungeon themed to them, with it split into four areas, each with a different elemental type in it and the heroes having to defeat the Elementals/solve a themed puzzle to gain access to the controlling sorcerer/Necromancer/Aether elemental/etc’s tower at the centre of the dungeon so that they can defeat him (sort of like the Crystal Maze).

You could have a puzzle involving teleporter pads, either ones with set destinations or ones that teleport to a random pad.

Another non-combat puzzle you could use is have one of the numerous variations on the classic knights and knaves puzzle.

More of a dungeon feature than an actual encounter or puzzle, but what about a collapsing dungeon? The heroes fight their way through to the boss/treasure and once they’ve defeated him/nabbed the loot the dungeon starts to collapse and they have to race out. This could include things such as chunks of roof falling into the dungeon (either onto heroes/monsters or empty squares blocking them), the floor collapsing (creating obstacles the heroes have to go around or jump over, or if using a multilevel dungeon, depositing heroes/monsters onto lower levels) and even parts of the dungeon getting blocked off or removed (possibly meaning that heroes miss out on loot if they didn’t nab it from an area on the way in. A possible variation on this is having a timer and having rooms removed from behind the heroes as the leg it, forcing them to keep fleeing rather than dawdle).

Similarly, how about a flooding dungeon where the heroes have to fight their way up the levels as the water rises. Could even have some parts where the heroes have to swim through submerged tunnels to get to switches they need to hit to advance.

Another puzzle you could have is maybe have it so that the heroes have to set off a particular trap to advance, but the trap is either malfunctioning or the weight needed exceeds that of the party combined, so they have to lure a big monster onto it to set it off. Alternatively, drop the part about needing to set it off for them to advance, but have the dungeon set up such that one way to kill the big beasty is to lure it into one or more of the dungeon’s traps so that they have an alternative to hacking and slashing to kill/bypass it.

Building off your idea of the dragon and the Behir, how about a dungeon with two factions of monsters (eg goblins and kobolds) who will fight if they come into contact and the heroes have to draw them together so that they can sneak into their lairs to nab two macguffins.

Talking of sneaking, don’t know what monsters would be suitable, but what about an encounter where the monsters are blind, but have very good hearing (and/or other senses) and the heroes have to try to sneak past, but putting a foot wrong will alert them. But similarly the monsters can be distracted by throwing a stone in the other direction.

Not an encounter per say, but how about a dungeon/room where the floor is covered in ice and because it’s magic ice, anyone/thing walking on it slides and keeps sliding until they bump into something solid (eg wall, pillar, etc) unless they have a special ability. Could also have a room where instead of a floor there’s a sheet of ice and anyone/thing walking on it is in danger of the ice cracking underfoot an collapsing, causing them to fall.

Talking of falling if you put a foot wrong, how about a puzzle where the heroes have to traverse a chasm by a narrow winding path, but an illusion had been cast over it to look like there’s a solid floor for the entire room. The heroes have to figure out where it’s safe to step, and where stepping will lead to a fall.

There’s also the classic escort mission – they have to get in and get the princess out safely. Or they’re escorting a dwarf merchant and his caravan through an underground shortcut and have to fend off goblins and stop them from making off with too many kegs of Dwarven ale.