Cardboard play structures work great! Make sure they have lots of chew toys so that they can be distracted and not chew on the cardboard though.
Also, my guy loves to play on me haha. He'll climb on my back if I'm laying down and whatnot.. We do "cuddle cave" where I cover myself in a blanket while I'm sitting and it makes a "cave" for him to explore haha, that game's a favorite. It's the only way that I get him to be a little cuddly, plus he uses it as a break between running and jumping. Just make sure you have enough ventilation though!