Make sure your rabbit has a shady spot to retreat to, and plenty of water to drink. If you are traveling or underestimated the heat level, get your bunny to a cool spot as soon as possible. Drape wet towels over their cage or carrier, or freeze bottles of water and place them around the rabbit. I like to keep a few of these on hand year-round for making quick trips in the car more comfortable. Attack by predators is another danger of the outdoors.
Some rabbits can be so scared by the mere presence of a predator that they die of fright.
Wild animals such as raccoons, owls, possums, etc. Don't think that just because you live in the city, wild animals don't visit your yard. I have even seen hawks in my urban neighborhood. Never, ever leave a rabbit unattended in the yard. Keep them on a harness and leash, in an enclosed pen, or within a few feet of you at all times.