Pets such as tortoises, guinea pigs, and rabbits enjoy time spent grazing and wandering outside, but it's important to keep close tabs on their whereabouts to prevent them from getting lost. You can obtain a rabbit leash at many pet stores (be sure it's the step-in variety rather than the standard model that wraps around the chest), as well as play yards that keep guinea pigs enclosed in a circumscribed space. A simple helium balloon secured to a tortoise's shell -- not tied around the neck or belly -- allows you to see exactly where he or she has wandered.
If you treat your lawn with fertilizers, herbicides, or insecticides, you should never allow your pets to wander over it; they should only be given access to a completely organic lawn. And be certain that they have some shade if they're outside on a sunny day.