I haven't taken my rabbits outside since I moved into the city, but before when I did take them out I built them a gaaint NIC pen with a roof and let them run around outside in that.
I always stayed out and supervised them while they where outside, but the roof on the pen was because there were a ton of hawks in the area I use to live and they where infamous for taking small pets even right in front of people sometimes. With the NIC pens, you can either not put on a bottom, or use the panels to make a floor. The floor would keep them from being able to dig out of the pen, but they can still access the grass to eat it (make sure you don't use any pesticides or anything on the lawn if you're going to allow them to eat the grass).
That always worked well for me. I didn't trust my bunnies enough to let them run free in the yard (Plus I was paranoid about the hawks) and I couldn't use a harness for one of my rabbits - so the pen was a good alternative. Just make sure your rabbit always has access to a nice cool shady area and fresh water, in case they get hot/thirsty running around outside.