>>33022355Two things: good dungeons, and optional/hidden areas.
A "dungeon" in the case of Pokémon isn't just limited to a cave but can be almost interior as long as it has some complexity to the map layout. Places like Tin Tower and the Abandoned Ship are dungeons even though they're not caves.
Optional and hidden locations are self-explanatory, but for a good example, see the northern half of Hoenn Route 115 (pic related).