Still fleshing it out but so far I know...
A bunch of cursed jewelry. They are in display cases with labels, but one of the kobold minions mixed up all the labels.
A room full of mimics and bugs that look like coins
A flooding room filled with giant sea anemone, telepathic sea-stars, and a giant shark bowl ooze (from Creature Codex)
A sculptor who creates life-like statues of people in terror and always wears a veil to cover her face, but who is not a medusa, just a really good sculptor being held prisoner.
A hall of deadly traps (to be determined)
A perfectly ordinary room just to make the suspicious.
Somewhere in there I'm going to include a sphere of annihilation.
And when they finally defeat him, the heroes discover that the dragon only really has one (non cursed) treasure: a deck of many things.