The Tacoma area has numerous beaches, and they all have their merits, but for wading, playing in the sand, or skimboarding, it's worth the drive to the Southwestern edge of Federal Way to Dash Point State Park.
Dash Point is a gorgeous park with a little bit of everything: beach, beautiful picnic spots, hiking trails, and camping. (First come first served this Summer, due to construction.) We made an unscheduled visit yesterday after meeting a fun new playgroup in Federal Way. The kids played in the ice cold creek that flows into the warm, shallow waters of the Sound, and waded up to their knees in the waves. My friend's toddler contented herself with taste-testing the sand that ran the length of the beach.
But you don't have to be under 10 to appreciate this beach. Because the water is so shallow you can wade out almost halfway to the horizon and still touch the bottom, it's a favorite spot for skimboarding, a sport that involves jumping on a small, flat round board and skimming along the surface of shallow water. I've heard this is a great sport for beginners and I'd love to try it out some time. When we visited yesterday, there were twenty or thirty skimboarders on the beach.
Getting there doesn't have to involve I-5. I took 99, but Dash Point is also accessible by taking 509 across to Northeast Tacoma, the best way to go for great views along the way.
Dash Point State Park