Guinea Pigs need exercise to stay healthy, it's quite hard to do this as a lot of things for sale, they can't have such as Runner Balls and Wheels because their spine curves a different way therefor it would damage their spine. Their back legs are very delicate too and rounded surfaces can hurt their legs.
Floor time is a great way for your Guinea Pigs to get exercise, in my picture you can see one of my Guinea Pigs on the floor, having floor time. Floor time is when you put a Guinea pig on the floor and allow it to go wherever, but you'll need to watch out for wires and cables as they may chew these, also things on the floor, they might pick it up and eat it, so watch out.
You could put tubes on the floor to run through and things you can get in pet stores to put on the floor such as balls. You should also block places to go such as under things and away from cables and close doors.
In the summer, an outdoor run like the one in the picture below (this is my run), is great, it is 11ft long, and 6-7ft wide I think. They provide lots of exercise when they run around in it, and when in pairs they can have great fun together as the space is very big. I have a hutch in there which I open the doors on and the Guinea pigs can come out, unfortunately, when we moved to our new house there was no grass, but the concrete slabs are a great way to keep their claws down.