My second son is 2 and he still loves his Plan toys cone stacker that got handed down from big brother!
Along those lines I'd say... http://www.atoygarden.com/index.cfm?...roduct_ID=1893
this fish and boat stacker is pretty neat.
The fish only make a perfect sail when they are placed in correct order by size *and* laid with the flatter side of the body down first... so it can be a challenge but only as much of a challenge as the child chooses... and the fish are useful as props in lots of other play, the boat as well. (just so you know, for some reason there is a gap between the top fish and the end of the mast, the mast sticks out a bit and that bothers me but not enough to really detract from the toy)
Guidecraft feel and find is something I bought recently that both my 2 and 4 yo love in diff. ways. I think it is great for something that can be adjusted to increase the challenge and present multiple play possibilities. The tiles the animals sit in are wood pasted on some type of fiberboard BTW, not quite the quality I expected, and the cutouts don't match the shapes as well as I would like but I am really happy overall.