Hwy 20 is the only hwy that runs through the Park. Ross Lake Overlook off Hwy 20 is the highlight (view of glacier fed lakes and snow-capped peaks). Washington Pass (rest area with a short 1/4 mile trail), on the east side of Cascade NP and outside park boundary, is another great place with very very dramatic views.
The most beautiful part of the park is accessible by hiking/back packing and boat only.
Hwy 542 is another scenic drive but outside of N. Cascade NP.