numbers! DS3.5 is (and has been) obsessed with numbers since well before he was talking, (granted, he didn't speak till 2) but so many games are just permutations of numbers...legos become buildings of numbers, he makes numbers out of playdoh (rainbow colored, of course)...and so on and so forth.

he has also very recently become interested in chess - he has no real idea of strategy right now, but he is enjoying moving the pieces correctly and capturing daddy's pieces.