Favorites in the North Cascades
DBCooper,
Colonial Creek is about it past New Halem and still this side of Washington Pass. There are a few National Forest campgrounds between Washington Pass and Mazama. Other than the state parks the only others are Marble Creek on the cascade river road outside of Marblemount (site #9), Howard Miller Steelhead Park in Rockport (has power and water), and perhaps the Puget Sound Energy campground (Kulshan) at Baker Lake - nice big sites and fairly cheap ($7 last year).
If going over the pass isn't in the cards then you could try Douglas Fir campground on the Mt. Baker Highway (stay in the section closer to the highway - it's down at river level. )
Or, just east of you on the Moutain Loop highway Red Bridge is nice (site #5), the wife likes Turlo (Site #15). Or go over highway 2 and stay outside Leavenworth on the Icicle River road, several campgrounds out that direction. On this side of Hwy 2 there is Money Creek, Bekler River and some others.
Take a look at ReserveUSA.com for reservations of National Forest campgrounds.