I'm guessing the nay sayers to outdoor guinea pig enclosures have never been to a guinea pig friendly petting zoo... In any event, guinea pigs can go outside and an outdoor enclosure can be a great place to build a nice big habitat for them but there is a lot to think about before attempting this. First is they'll need an enclosure a lot like you'd give a rabbit that has at the very least a box to get away from direct sunlight, wind, rain, and to hide in if they feel threatened by something. Also it'll need to either be a grated bottom cage like a rabbit hutch, allowing droppings to land on the ground outside the cage, or it needs to be a portable cage that you can move around when they soil it. These work the best for outdoor habitats. Guinea pigs love the grass so I prefer the second option myself but that's really up to you and your piggy. Most guinea pigs aren't big on digging but they're all individuals and ones who do like to dig pose a risk of escape so a grated bottom cage would work better for them. Always keep your eye out for these sorts of things!
Also there are play pens, and rotating cage tubes (somewhat like hamster balls) you can look into but you have to supervise them with these so they won't get harassed or get into any accidents or roll away.
Things you have to consider before setting up an outdoor enclosure is the weather, predators, and parasites. All pens should have a top! Otherwise guinea pigs can be free lunch for hawks, other birds of prey, neighborhood cats, wildlife, and maybe even picked up by unsupervised children. I wouldn't recommend keeping your guinea pig outside at night... simply because that's when the clever and determined foxes, fisher cats, and other potentially dangerous things come out to eat. I also would caution you to consider the weather. Guinea pigs are pretty hardy but it's probably not good for them to be out in extreme cold or heat. And guinea pigs can get parasites, of particular interest are mites, ticks, and things like ringworm. There are preventatives and cures for all these but it's best to know them beforehand. Use common sense and you could end up with a great new environment for you and your piggy to play in.