Smart! A real sandbox can be quite an investment (and eat up a good chunk of your yard), but here's another option.
Those clever Parent Hackers thought to use a rolling, plastic underbed storage box as a sandbox. You can find one at larger hardware stores or at big box stores like Home Depot, Kmart, etc. (here's one for $15 at Lowe's for example). Fill it with sand or dirt and then roll it out of the way when not in use. The cover will keep the sand clean and dry.
(Via: Parent Hacks.)