Teach him/her multiplication! Probably not the most exciting thing for every child, but I was so proud of myself when I learnt it. You probably have to trick him/her into wanting to learn it. When I was 3, I was with some friends in the sand at a park, and this "big kid" came by and asked us what 1*1 was. We all said two. She said we were wrong and made fun of us.. So, I asked my mom what she meant, and the first maths problem I remember solving on my own was 8*8. (That was a lot of counting on my fingers!)
But yeah, play board games (heh, I have like 50 in my closest that take up sooo much space, but my mom won't let me get rid of them), READ, play make-believe, go for walks and stuff, do maths, teach him/her something interesting.