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.
Things (Fire hydrants, benches, drinking water fountains, bicycle parking, trashcans) and Places (opening hours, website and contact info, adding new local businesses)