If you would like your rabbit to be able to run around the yard or the park and you're worried that he might go too far, a harness and leash is a good idea to try. Below are some types of harnesses that work good for rabbits. It's better to use a harness rather than a collar because a colar would choke the rabbit too much when you want him to stop.
Types of Harnesses to Use
Pet stores sometimes sell harnesses made for rabbits. The kind with the thin rope may work best for smaller rabbits or rabbits who don't pull very much or get very excited when they're running around in the yard. If your rabbit is larger and likes to pull, a rabbit harness made with wider flat rope or a harness for a small dog (size X-Small) may work better. Harnesses are usually adjustable so you will be able to adjust them to fit your rabbit but it is best to check a head of time to make sure they can be made small enough or large enough.
I also sell rabbit harnesses that I make out of fabric. To see them click on the link below. If you would like to see the other rabbit themed things that I have for sale, click on the "gift shop ~ rabbit accessories" link on the left.