North Cascade National Park is gorgeous in the summer and not overly crowded like other parks in the PNW. It has many turquois lakes there but one of the prettiest is the Diablo Lake. The hue is so unbelievably blue/green that it looks unreal that one must see it in person. Diablo Lake look out is just off the road requires no hiking to view it.
Washington Pass is one stretch of road in Highway 20 that is windy and has gorgeous views of peaks, such as Liberty Bell. At the Washington pass lookout one can view from above of how beautiful the park is, without much hiking needed as well.