Yeah in a castle you have just a floor sometimes filled with water or lava, a celling and walls.
With a free boss environment, you could have various things, like if in a forest area, there could be different tree trunks that you have to jump on with a few enemies on them, then you get to a big tree for the boss battle, where the boss could be a piranha plant, that uses the giant tree to hit so it obtains coconuts to throw at Yoshi ect.
Endless possibility's, for a blooper boss it could be a few sunken doom ships, that fire bullet bills, bomb-ombs and cannon balls, the blooper throws wreckage at each ship to hit Yoshi/or sink the ship then once you get to the main ring made up of ships the fight begins, after being hit the blooper can break of two pieces of the ring, and some cannons can fire at Yoshi, then the fight continues.