About 15 miles west of Cle Elum [1] just off I-90, Lake Easton State Park encompasses 518 forested acres on the west and north sides of Lake Easton and is a popular camping spot (if you can ignore the traffic noise). The mile-long reservoir is good for swimming, boating, trout fishing, and water-skiing. The water level changes through the year in this reservoir, and it is drained each winter to provide irrigation water for eastern Washington farms.
Hiking, cross-country skiing, and snowmobiling are popular dry-land sports. Kids enjoy the big-toy playground.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/cle-elum-area/cle-elum