Personally, I like to micromap. I’ve mapped individual trees, poles, benches, and trashcans, for example. I also like to map forested/scrub areas. Buildings themselves are also kind of fun to map, so long as I have the building plugin installed to JOSM.
My favorite is marking rocks in the sea. I live in an area with a rocky shoreline. I wish it was legal to chart the seafloor for depth.