A cave or dungeon doesn't necessarily have to have a big puzzel. If it's well-described with great details then you can still get the reader involved.
So... If it's a dragon's cave, I would imagine it being like a labyrinth... You know, the twisting tunnels carved into the rock. But the way the tunnels are formed is obviously unnatural. Maybe the dragon that lived there made the tunnel by digging so you can see where huge claws raked out the rock. Dragons also have a tendency to collect gold and other treasures, so there's piles of treasure here and there or a giant room of it near the end. There could easily be pools of murky, green cave water. Underneath the layer of moss over it could be more gold or perhaps the skeletons of this dragon's victims. And, of course, dragon eggs (either broken or yet-to-hatch) would likely be in aplenty.
Ooh! And let's see- Instead of a puzzel- the great challenge could be the dragon that lives there! Maybe your characters are going through the cave and suddenly see the dragon, but it's sleeping. They have to sneak around it to get to the exit (or whatever). Problem being that the dragon twitches and thrashes in its dream, so they have to avoid its whip-like tail, giant feet, and sharp claws- all without making a sound. You could even have the dragon open its eye in its sleep for extreme tension, but have it not see anything and continue sleeping. Or, it could wake up (or be awake the entire time) and the characters would have to fight it. :D
I'd have to think on the mansion more... Are your main characters Pokemon or Trainer? (I'm thinking Pokemon like in the Mystery Dungeon...?)