Find rest and repose by the shoreline at Pearrygin Lake State Park (five miles north of Winthrop, 509/996-2370 or 888/226-7688, www.parks.wa.gov [1], $17 tent, $23 full hookup, $7 extra reservation fee), which offers lakeside campsites mid-April through October. This is the largest lake in the area, with lots of space to fish, swim, boat and even water ski.
The Forest Service maintains a dozen campgrounds ($10) within 25 miles of Winthrop [2], and most other areas are open to free dispersed camping (with the exception of the Highway 20 corridor). Closest are the campgrounds up West Chewuch Road: Falls Creek, Nice, Flat, and Buck Lake. Falls Creek has the added attraction of a beautiful falls just a quarter-mile hike away, while quiet Buck Lake has ducks and cattails.
Other often-full Forest Service campgrounds are west of Winthrop along Highway 20: Early Winters, Klipchuck (a very quiet spot), and Lone Fir. Most of these campgrounds are open June–September. Get more information from the Winthrop Ranger District office (509/996-2266).
RV campers can also choose to stay at Big Twin Lake Campground (210 Twin Lakes Rd., 509/996-2650, $27 full hookups), a private property right on the 90-acre lake offering rowboat rentals ($25 per day), fishing, and free wireless Internet.
Links:
[1] http://www.parks.wa.gov
[2] http://www.moon.com/destinations/washington/north-cascades/methow-valley/winthrop