I found these fun tropical mats at Target in the Dollar section awhile back. I didn't have anything in mind for them when I bought them I just thought it was a great deal.
But on days when Nate needs something to do I'll break them out and we'll do Tumbling. He'll hop from one mat to the other.... skip, jump, somersault, roll, you name it. Obviously you could do it without the mats, but for $4 I think its worth it - it helps get and keep his interest. He thinks its the coolest thing. And as a bonus it helps wear him out!