When my daughter was three, we bought an atlas, a childrens dictionary, a children's encyclopedia, ect. Kids really want to know about everything. I just listened to the questions my daughter was asking, and taught her based on what she wanted to know. The encyclopedia was as much for me as it is for my kids because when I didnt know the answer to a question we could look it up together. Also, just keep your eyes open and you will be amazed at what you see. If your son is looking at an ant hill, then you can tell him about ants. We really like Insectlore.com, which is a place where you can get caterpillars which turn into butterfly's (we even bought an anthill). Making tracer pages with simple shapes is a good way to get children ready to begin writting letters. Uhm...animal flash cards and letters and numbers were a lot of fun too. Keep it light and fun, and then you will be amazed at what he learns simply by talking with him...Good luck!