Admire the setting. Particularly pretty in the Fall with the leaves turning.
For the under 5's, do the easy 2 mile loop around the lake and then on a warm day (which is rare in these parts even in summer), take a dip in the lake.
If your kids are older and are seasoned hikers, attempt the 4 mile round trip hike up to the ledge for a 270 degree panorama of the surrounding mountains and lakes, watch the fog drift below you, pretend you are on a cloud. It is 1200 foot elevation gain but sweet reward!!! Bring lots of snacks and water along the way and having got all the way back down, go jump in the lake!
To me, the beautiful cool summers of the Pacific Northwest are all about hiking and camping and eating the wild blackberries on the trails along the way. A great resource for easy hiking trails is the book: Best Hikes with Kids. Western Washington and the Cascades.