1 plastic storage bin
plastic cups (5/$.99 @ Target)
a few bath toys, measuring cups, funnels, etc.
Mix ingredients in the plastic storage bin. Fill halfway with water, set on a table outside and you have a water table for at last half an hour of kid fun! (Keep a towel nearby for those cuties who tend to be creative with where the water lands.)
Fill halfway with sand instead of water. Not quite as messy as a sandbox but can be just as fun!