Summer use outdoors require you to keep a portion in the shade, provide water and also check to ensure your pet doesn't become overheated. A quick check how warm your bunny's ears feel before placing him/her in the pen and periodic checks are mandatory. If you see the bunny panting or getting moist around nose, time to get him cooled down.
Make sure it is set up sturdily so as not to fall over if your bunny decides to a an extreme sports binky. Keep other animals away and if your area is known for flying predators provide protection. All in all, as you would a small child, do not leave the pet unsupervised.