I would suggest a good set of wood unit blocks; not the junky kinds from Toys R Us that are too light and topple right over. Something like this http://www.liveandlearn.com/woodtoys/unitblocks.html

They are expensive but at our house have been the most long lasting toy - every day from age one to ten. They can be passed down to the next generation. Wood blocks build spatial skills and fit well with open ended imaginative play. These are a preschool standard for a reason. The weight makes them easier and less frustrating to handle than plastic toys.