Some guinea pigs love being outside; others are nervous about open spaces (and strange noises and smells). Don't trust your guinea pig to stay next to you; make a small play area out of wire, short fencing, or wire shelving. You can use anything, as long as your guinea pig cannot escape! Place something for the guinea pig to run into when frightened. One of my guinea pigs heard a crow in the distance one day, and just ran in circles from fright! I put a small tree branch with leaves in the play area, and she dove under it for a few minutes.
Guinea pigs are adorable munching happily on grass, basking in the sun and making happy wheeking noises. Only take them outside on warm days — if you can wear short sleeves comfortably, then it's warm enough. The ground should be dry, never damp, and the area should be free from pesticides, animal droppings and chemicals. Do not leave your guinea pig in full sun; it will get overheated too easily. It is essential to provide a water source, either through slices of watermelon or other watery fruit, or a water bottle. Once you've made sure that your guinea pig is comfortable and happy outside, enjoy yourself! Just keep a close watch.