Google Froebel and check out their site. I really love their inspired toys. We picked up a set of small Froeble blocks for our 3.5 year old son; this set in particular is small and comes in a box. Our set is inspired by Frank Lloyd Wright's Prairie House, and comes with directions for building it; we just encourage our son to do basic building with his and don't have a specific end product. Another neat thing about this 'toy' is that because it's portable, it can go out with you, and is something other adults, I've discovered, like to play with too.
As a preschool teacher, I've noticed that there's something very welcoming about blocks to children. Little blocks also feel safe to experiment with for some kids, because if they come crashing down, it's not as dramatic. Our little boy loves these blocks and will sit for 20 minutes or more to play and explore with them.
Good luck on whatever you choose. I'll be curious to see what other mom's recommend.