This is a great age to introduce art and science activities.
Art:
Finger painting (messy - save for a weekend when you have clean up time)
Make instant vanilla pudding. Divide into 4 bowls. Mix in different food colors. GIve him some paper & let him go wild! Be sure to put him in old play clothes, as this can stain. But you don't have to worry about him eating the paint!!
Macaroni Master pieces
You need Elmer's glue, different shaped macaroni noodles, paper, and paint. Help him glue the macaroni on the paper. Have a snack while it dries. Then let him paint his master piece.
Science
Heavy & light
Take a container of water outside and help him gather stones, sticks, leaves, whatever. Then let him put them in the water to see which are heavy and which are light. My son loved this at that age.
Sweet & salty
Make up several types of little snacks, some sweet and some salty. Blindfol him and tell him he has to tell you which are sweet and which are salty to get the next treat. Sounds silly, but again, my son thought this was hillarious at that age.