Trio's (Fisher Price) are a similar toy to legos, aimed at the 3-5 y.o. range. The lid to the box is a board that the pieces snap into to help build up fun stuff. We don't even really have than many of them (no more than 100 pieces) but the variety of thing that my kids come up with is amazing ... factories, airplanes, cargo ships, flashlight-holders, trucks, trains, farms, houses for small stuffed animals, etc etc etc.

And you'd be surprised at how much fun 3-4 years old can have with laundry baskets and small blankets. Throw in a few medium-sized cardboard boxes from U-haul and it's pandemonium.