We like Steamboat Rock. It's near the Grand Coulee Dam. There is Lincoln Rock too. It's near Wenatchee. Been to Wenatchee Confluence a lot too and really like it. We seem to go to the Eastern side of the state in the summer anymore. We do like Pacific Beach on the coast too, but it's always raining there... Well, at least all the times we go!
All these are state parks, so if in the summer and over a weekend, you will need to make reservations well in advance..
Good luck!
Mitch