Я Городная Мышь. ;-) "Outdoors" is where bugs live.
Moreso, actually. I flagged this cuz I wrote "in" and it wasn't accepted. I never say "on" the grass unless something is actually on top of it, such as spray paint for utility marking, or a blanket. Groceries and a ball would be mixed in with the grass, so this should be "in".
I agree, in the grass is more of "in the area with grass," "on the grass" would either be dew clinging to grass or "standing on the grass" as in standing on ground that happens to have grass, although "standing in" works for taller grass. It's contextual, but "in" is most common around me, midwest America.