What single element in a dungeon sends characters running towards it more than any other? That's right, a nice, locked chest. Of course, these things are often trapped or end up being mimics. So, while the players may want that loot, they are going to poke, prod and double check that chest before they open it. So, let's not have the chest be a trap at all. Instead, the chest is protecting the players from the trap.
A sturdy, locked iron chest is chained to the floor.
Type: mechanical; Perception DC 24; Disable Device 29
Trigger touch; Reset None
The chest covers a volcanic vent. When unlocked and unchained, lava spews forth. Lava Attack (6d6 fire damage, DC 24 Reflex Save for half damage); multiple targets (all targets within 30')
If nothing happens when they unlock it, they may be confident enough to just let the lid fly open, at which point they should be unpleasantly surprised. After this one, you probably aren't going to have to bother with trapping the rest of the chests in the dungeon, I'm sure they'll spend plenty of time examining them.
Just make sure you have a good back story for this one. Perhaps this is the garbage disposal for the dungeon or an area that the blacksmith uses. Maybe later on there can be another chest, and a kobold carefully opens it up and tosses his trash in it as the players observe.