I loved LEGO as a kid, and I still love it now. So does my son. It's a bit pricey at first, but kids play with LEGO for years, and that longevity makes it a great value in the long run.
Right now take $5 off a $20 purchase at LEGO.com with code bscd28 when you shop through Upromise (exp. 7/1). You can also earn 3% for college.
You can shop by age at LEGO.com, which makes it easy to find appropriate sets. I recommend the LEGO Large Brick Box for kids four and up. It's $29.99. Parents of younger kids can pick up the larger DUPLO blocks at LEGO.com as well. Also be sure to check out the "Sales and Deals" area for savings of up to 50% and more.