Calm fun things to do with kids?

My nephew is staying the weekend with me because both of his parents are working. He can't go to the babysitter because his doctor said he needs to rest until next week when he goes to the neurologist. He has really bad headaches. What can we do that doesn't involve running around, jumping on the trampoline, etc? I just need some ideas for calm stuff we can do cause usually we are outside playing sports.  He's 6 years old.