To encourage a pet guinea pig to exercise, you can provide them with a large cage or enclosure to move around in. You can also place tunnels, toys, and safe items for them to climb on or explore. Additionally, you can create a playpen area outside of their cage where they can roam freely and explore new spaces. Guinea pigs also enjoy interactive playtime with their owners, such as supervised floor time where they can run around and play. It's important to provide a variety of opportunities for exercise to keep your guinea pig healthy and happy.