Grand Coulee Dam -- 2 or 3 nights -- Where

So much to see in that area.

You could check out Washington State Park - Steamboat Rock as well. They have a really nice campground for fees, but also several 'satellite' camping areas that are available as well without fees. Those are located just outside of Electric City, on Banks Lake down to the park entrance. See the map at https://washington.goingtocamp.com/ for more details about this park.

Lake Roosevelt NRA Spring Canyon. Few miles east on WA 174
Scroll down to Spring Canyon
Lake Roosevelt

https://freecampsites.net/#!Coulee+Dam,+Washington,+United+States

Very handy software.