The dragon is a pack rat and has filled the cave with completely useless stuff.

Some areas are actually packed to the point the dragon can't get in.

Nasty creatures (that are not nasty to a dragon) live in some of the detritus.

So, the "collect pirate ships" -- there will be a cave filled with pirate ships. Like, top to bottom. Space filled. With some jammed in and broken. The entire place is a maze of pirate ships.

Those pirate ships contain treasure maps, pirate booty, etc. The dragon always meant to fish it out, but got distracted, and put more pirate ships on top.

Oh, and the dragon initially flooded the cave by opening a gate to the plane of water, so it is colonized by water elementals creatures. And one ship had a pair of breeding mimics, which have spread over most of the place.

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A garden, full of plants that the dragon found interested, decorated by various garden-statues. Many of the statues are gargoyles, many of the plants are hostile to mere mortal life.

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Pet owlbears. The dragon has been breeding them for a few 100 years, and has managed to get them to play fetch.