CASTLES, DRAGONS, FACES, WHIMSY: Take a bucket. Fill it with damp sand. Turn it upside. Stick a twig on top. And, you're done. We've all likely tried our hand at building something out of sand, but we'll wager that most people haven't constructed full worlds, complete with trains carrying happy turtles and giant, realistic heads emerging from the beach. That's exactly what goes down at the U.S. Open Sandcastle Competition, which is marking its 30th anniversary in Imperial Beach in 2010. The categories include castles, best sculpture, and creatures of the sea. TEMPORARY BEAUTY:Saturday, Aug. 7 and Sunday, Aug. 8 are the dates. Imperial Beach is about a half hour south of San Diego. And remember, this is one of the few art competitions on the planet where shoes are optional. This is also one of the few art competitions in the world where the art, afterwards, must remain only in photos and memory.