I came across something that looks simple, but fun...it's Gift Venture.   They send your kids three mailings that follow a "story" and lead them to a "gift" that YOU hide in your house somewhere.  I'm going to try it for the kids with the Valentine's one "cupid".  I'm going to just let them think this arrived in the mail, and of course I won't let them know that I was the one who hid the present...if it's good, I think it would be fun to do on a kid's birthday!