There should be tons of downhill and extreme mountain bike riding in the North Cascades, but there's not. There is a lot of cross-country riding, and so we do cover that here.
If Granite Falls, WA can make plans to put in a motocross track, there's no reason property owners alongside the foothills of the North Cascades can't put in an extreme downhill track for mountain bikes.
Hint: There are tracks out there, but they're privately owned and you have to know someone in the know. If you spend time on the forums of Ride Monkey, you may get a hook up from someone with some connections.
Cascade River Road - Cascade River Road is a logging road with access to North Cascades National Park. While biking isn't allowed on trails within the park, the large number of logging roads in the forest service lands outside the park provide excellent cross-country mountain bike riding.
If you map out your trip in advance, you can choose roads that cross creeks and rivers, as well as roads that will take you up to ridgelines, where you're sure to find excellent vistas.