I have two 3 year old sows who are exactly the same. When they were younger they used to zoom around during floor time, and would popcorn and explore. As they get older they become less active.
Make floor time different each time. Put out lots of tunnels, boxes, houses, beds, and make a fleece forrest for them to run through. Also put out a few different types of toys (balls, jingle toys, soft toys, toilet roll tubes stuffed with hay) for them to play with. Make it as interesting for them as possible. Feed them their veggies during floor time, but don't just put them in a bowl, scatter them around the area and hide them so they have to go searching.
Don't worry too much about it affecting their health. Guinea pigs aren't active animals. As long as you are feeding them an appropriate diet and giving them the opportunity to exercise by letting them out for floor time every day, then they will be fine.