Buying some LEGO Baseplates when they were on sale was the best decision I made for my LEGO loving kids! You can score these 10×10 green LEGO Baseplates for just $5.69 (reg. $10). That is a savings of 43%. These don't go on sale too often, so if you want some I would grab them now.
If you have a collection of LEGO's but don't have any baseplates, I would highly recommend buying 4-6 plates at this price. I have found that my kids build more and will build larger projects now that they have baseplates. Sometimes they also like to display their creations for a while so having multiple plates has been very handy for us!
Another super easy project that LEGO loving kids will enjoy, this LEGO Lap Table that I made for my son a couple of years ago. Just glue one of the 10×10 baseplates onto this plastic Lap Tray from Michael's (often on sale for about $5) with some E600 glue and your kiddos have a great place to build and store the LEGO's for their current project!