Google things like "crafts for pre-schoolers", "crafts for toddlers", "pre-school activities",
Get a FamilyFun magazine...it has tons of things to do with kids...I love it.
Go to the thrift store and look for books on things to do with kids.
Let him help cook meals and bake things with you and or his babysitter. Even get him a little apron that he can decorate.
Have him finger paint.
Put him in the tub with tub crayons.
Get him a Big Bucket of Leggos
Get him kid friendly games for the computer...my oldest learned to use a mouse at that age.
Go to the library and get educational dvds like Signing Time, Leap Frog and so on.
REad to him.
Get some sing-along-music