>This land is grassy and looks to be regularly mowed,
If used for nothing else (eg not a `landuse=recreation_ground`), `landuse=grass` suffices. We would like to describe what's on the ground foremost. You could try tagging the official classification/purpose somehow, perhaps somehow. Perhaps `designation=`, if you want something more than `description=`.
(This assumes it is only a buffer / green belt, not a `boundary=protected_area`)
>with many trees.
Now this depends on how "many trees" there are. By the sound of your question, it doesn't seem that many, so you can leave it to the next step.