Hi, and first, thank you for Rescuing the guinea-pigs, there are so many out there waiting for a home.
Boars can be bigger (our neutered boy weighs 3lb , but claims it's because he's a young Rex at the prime of his life, not because he's an eat/sleep/pooh machine)
If you can arrange a run ( C&C style cage that you can fold up or a childs travel cot or a paddling pool. All will give them a new area to explore and you can store them away)
Tunnels - plastic ones (Pets at Home or online ) We got Hopp Inn tunnels (but not the cube, the door is quite high) Cardboard boxes with doors in/out cut out Paper bags (ours like Primark bags but cut the handles off - memory of one of our boars getting stuck and raging )
If you can get the big tubes from carpet rolls you can cut them up (ask at your local carpet shop if they have some, measure first though to make sure he'll fit)
We got a terracotta drain pipe from B&Q but it got broken. Our pigs loved it, cool in summer.
Put some small heaps of hay or treats and they'll mosey about exploring . Change everything round from time to time to confuse them and keep them interested.
Apart from that, they don't excercise apart from their jaws going side to side
I have just set up out large indoor run in the conservatory for the winter. In that they have tunnels, boxes and paper bags filled with hay. Before I set it up and during the summer if its too wet to go out, I just throw a couple of old towels on the kitchen floor and put the pigs on them with a box or tunnel. My boys are lazy and not overly blessed with the adventurous spirit/brains, so they won't leave the towels to walk on the wooden floor. I have left them on towels all day and they've never strayed.
Ours don't like tiled floors , actually it woks in our favour. If DD needs to leave the pigs , she puts them on the kitchen floor - I;m MNetting at the table so I ca watch them- and they don't move . Not an inch. They just sit there looking like Father Dougal till she comes back