Use old towels to make this huge beach blanket. Add a vinyl tablecloth to the back: blocks sand, doubles as tablecloth. Road trip must-have. To be able to machine wash the blanket, I would use Velcro to attach the tablecloth to the towel/blanket.
Drip Castles are a fun twist on a traditional sand castle. Fill a bucket with half sand and half water. Pour wet sand through a plastic funnel to build an abstract beach structure. Don't have a funnel?