Park Service campgrounds accessible by car on the North Cascades Highway [1] include Goodell Creek (518/885-3639 or 877/444-6777, year-round, $10), Newhalem Creek (518/885-3639 or 877/444-6777, www.recreation.gov [2], mid-May to mid-October, $12, $9 extra reservation fee), and the best choice, Colonial Creek (518/885-3639 or 877/444-6777, mid-May to mid-October, $12). All are on a first-come, first-served basis, with no reservations.
Brave the 39-mile gravel road from Canada and camp at the free Hozomeen Campground at the north end of Ross Lake [3], open mid-May through October. A handful of free campsites (no potable water) are available at Gorge Lake Campground (just north of Diablo on Hwy. 20). Enjoy summer naturalist activities at the Colonial Creek and Newhalem Creek campgrounds.
Along Cascade River Road, southeast of Marblemount [4], stay at Marble Creek Campground (518/885-3639 or 877/444-6777, www.recreation.gov [2], year-round, $8, $9 extra reservation fee). The free Mineral Park Campground is also open year-round.
On the east side of Washington Pass, you can camp at the very popular Lone Fir, Klipchuck, and Early Winters campgrounds managed by the Forest Service.
Links:
[1] http://www.moon.com/destinations/washington/north-cascades/north-cascades-highway
[2] http://www.recreation.gov
[3] http://www.moon.com/destinations/washington/north-cascades/north-cascades-highway/north-cascades-national-park/ross-lake
[4] http://www.moon.com/destinations/washington/north-cascades/north-cascades-highway/rockport-and-marblemount