How to tag small groups of trees, grassland, wasteland, and more

Use natural=tree to tag individual trees. Usefull when trees are isolated (in a city for example) or stand out for some other reason (very big or old or famous).

Land uses tend to handled by "landuse"; see taginfo for details about what values people have used. Up the road from you someone's added some "landuse=farm" around Denstone, for example.

Wasteland's a bit more tricky; depending on what it's actually used for it might be a recreation ground, brownfield, or construction

By "crossing" do you mean a railway level crossing or pedestrian crossings?

Re outbuildings, there are a number of suggestions here.