Right now you can get the following LEGO Ultimate Building Set – 405 Pieces (6166) for $15. If you have a smaller child, you can pick up the LEGO Duplo Building Set-71 pieces (5506) for $15 as well. These ship free with $25 purchase or free if you have amazon prime. but at this price it is easy to buy two and stash one away for birthday present.
It looks like the price has already gone up on these on Amazon. but you can also order them online from Walmart. Choose to pick up in store to get free shipping: