I’m sure you’ve already thought of this, but make sure there aren’t any pesticides or fertilizers on your lawn before you let bunny play outside. If you use these things, you should wait at least a week, and have one rain storm, before you let bunny on the lawn.
Also, make sure bunny has a clean bum. Flystrike happens to rabbits, too; it’s not just a chicken phenomena. One of the three main culprits of flystrike is the common housefly. Flystrike is gross, painful, and hard to treat. If your bunny has a dirty bum, the flies will be attracted to it.
Make sure bunny has some shade, too. Direct sun for long periods of time with no ability to access shade can cause two problems. 1) If your bunny has dark fur, his/her body will heat up in direct sun. Even if the weather is chilly, direct sun can cause a rabbit’s body temperature to get too high. 2) Bunnies, especially those with light colored coats, can get sunburn. Their eyes, just like ours, can also get sunburned.
That said, I’d recommend an enclosure big enough for you and the bunny. They are a ton of fun to play with while they romp outside!