In the wild guinea pigs have lots of aerial predators, and are therefore instinctively nervous in open spaces. This is why, whether indoors or outdoors, you need to supply hideaways - somewhere to run to if the GPs feel unsafe. Wooden hidey holes and wooden arches are possible additions to your run - you'll find such things available i the small mammal section of most pet shops. You can also buy tubes for them to hide and have fun in, such as our Zippi tunnels.
Guinea pigs feel comfortable if there's a handy bolt-hole nearby
When your guinea pigs are playing outdoors in their run, a hideaway is not just a place that they can feel safe, but a place in which they can shelter from the sun. Just like us, guinea pigs can get sunburnt. Providing them with a place to get out of the heat is a very good idea, especially if you intend on shutting the door to their hutch or otherwise restricting access.