If you are looking for ideas - I heard a GREAT one on the radio today - find a freind who is pregnant, and have tem do a pregnancy test thingy.
Leave it out on the counter, dresser...etc. Your DS wouldn't even need to be involved.
Another - EASY, PEASY one is to unscrew all the lightbulbs in "their" bathroom. DD and I did this one about 5 years ago. It was hilarious.