*My kids love to play with beans. I buy them in bulk, lay out a big sheet, and give them bowls, utensils, and cake pans. You could buy rice instead, if you're worried about choking.
*Get some of those bath crayons or paint, and put him in the tub.
*Play-Doh
*Make pictures with crayons/stickers for family members.
*I have big bins of toys and books in the garage that I bring out only when it's raining or super hot in the summer.
*Take a nap together.
I have 2 boys, so usually we just go out in the weather for a walk in the rain. They love it! I always put down an old sheet in the entryway and place some towels by the door before we go out. Then when we get back, the mess is limited, and everything gets hung up or put in the dryer.
Also, a great video for kids is Raffi in Concert. It's fun without being over the top, and the kids sing and dance along.
Good luck!