Start off the festivities with a quick project that lets guests use fabric markers to transform T-shirts into hip cover-ups they can wear later when they get out of the pool. (You can either pick up an inexpensive package of oversize T-shirts or ask the kids beforehand to each bring one to the party.)
To make the cover-ups easy to slip on and off, trim off the collar below the seam across the front and just above the seam around the back. Then cut off the sleeves just beyond the shoulder seams. You can even remove the lower hem and cut fringe around the bottom of the shirt.
Slide a piece of cardboard inside each shirt to keep the ink from seeping through to the back.
Let everyone color on their own design. To create the beach ball shown here, we first traced around a paper plate (a technique that works well for a sun, smiley face, or any other circular shape).
Allow the ink to dry completely (about a half hour) before getting the shirts wet.