Hi and welcome to binky bunny
If the two bunnies aren’t bonded, then I’m afraid they can’t have ‘play’ time. Bunnies don’t work like that, and will likely fight a bunny that they don’t know. If they’re both spayed and neutered there’s no reason why you can’t try to bond them though.
As for the balcony, as long as there’s no way of them falling off, supervised individual playtime sounds like a great idea
Good things to plant are any herbs (coriander, parsley, rosemary, basil), maybe a small apple tree (they love chewing on the twigs), lavender. Planting a tray of grass sounds like a great idea!
It might also be nice to give them a wooden box outside, that they could hide in if they got too hot.
The other thing to be aware of, depending where you live, is predatory birds swooping down and grabbing them, although hopefully if you were out with them the whole time that wouldn’t be an issue.