What activities can I do with my almost four year old who has his arm in a sling?

My son was at my parents house and fell off their slide, fracturing his humorous bone. He is now in a sling. The doctor doesn't want him to do any running and doesn't want us to take him to the park because he could fall and break it worse. That means that we're basically stuck in the house because anywhere new we take him he likes to run. We're thinking about taking him to the zoo and putting him in a stroller, but I don't know how well he is going to like not being able to walk around. Does anyone have any ideas on what indoor activities I can do with him. I have to mention that he is a little behind developmentally so games are out, he doesn't understand them yet. Any suggestions?