Hi there BandiTank here (formerly known as I Love My Aussie). I take my Guinea pigs outside quite often. Just make sure the grass (since they WILL eat it if given the chance) is not chemically treated. I don't even use a harness on mine because they don't move much, I just keep petting them, so they are assured and will be less likely to run off. My male loves kids and all the neighborhood kids come to pet him whe I o outside, but they can get a little scared sometimes. My guinea pigs always look SO bored so I take them outside often. Your harness is a good idea, especially since you don't know what they will do. I started out with mine ona man-made harness. Good luck!

-Rachel N Critters