Regular exercise is extremely important to your guinea pig's health and provides entertainment to fight off boredom. You can provide your guinea pig with a good deal of exercise as well as all-important handling and contact with you by playing with her on a daily basis.
Guinea pigs love to go for a romp outdoors, with proper precautions and supervision of course. Outdoor excursions can be another excellent way for her to get her exercise. However, an important way to make sure she gets enough exercise is to furnish her cage with a few accessories so she can keep herself entertained when she's alone.
Guinea pigs love to climb and explore, so try adding a few "ladders" made of bricks to her cage. These will provide her with places to climb, and the rough surface will help keep her nails worn down. Stack the bricks in a pyramid fashion to provide "stairs" for climbing, and make sure they are secure and cannot fall or injure her in any way. For example, place three bricks on the cage floor end to end, followed by a second layer of two and one brick as the third layer.
Once she climbs up to the top of her brick ladder, she'll need a way back down. Of course, she could climb down, but you can add a little variety by providing a "chute" for her to slide down. Use a piece of four-inch PVC pipe cut to an appropriate length, and secure one end near the top of the brick pile with the other resting on the floor of her cage. Presto! The chute will also be a tunnel, which your guinea pig loves. If you really want to get elaborate, you can use various four-inch PVC fittings, T's and elbows to join sections of pipe together for tunnels, sleeping areas and hiding places.
A little imagination and creativity can go a long way to provide your guinea pig with exercise and hours of enjoyment.