First--Isn'tThisFun-- thanks for the suggestion of using 'stampers' in playdough-- I actually have a bunch of ink 'story stamps' which don't hold ink very well but would be great to use on playdough. :)
I'd say blocks and flashlights (esp ones which can switch colors with different lighting 'gels') have held Kiddo's interest.
Asked him just now, he says he likes to play with 'food'. Go figure, what a joker. But I think he has a point, and some kid-sized kitchen utensils would be great. Or a cherry pitter. He has always thought it was cool.
Oh, and the marble run... I have been using that since he was about 2.5. He had a lot of small drums and liked to stack them on things, so the marble run was first built to become a 'drum kit' of sorts, and then has been used in it's proper form. This specific marble run is my favorite (in all my years working with kids with all different kinds of sets); you can remove a couple shorter, trickier-to-build pieces from the set when they are younger and add them in when they get older.
http://www.discountschoolsupply.com/Product/ProductDetail...
The price is good on this and a hint, if the loopty-loop comes apart, a blue produce rubber band works wonders.