Just past North Bend off exit 38 on I90 are miles of mountain biking trails. I haven't yet gotten the chance to explore them, but they looked fun! I read this about these trails:

Trail Description:
Exit 38 is a popular ride for the local downhill and free ride crowd. There are insane drops, steep technical sections and lots of obstacles to keep even the most seasoned downhiller on their toes.

I can't wait to go try them out!

Cool, thanks for this info Marie! I used to mt. bike all the time when I lived in California, I need to find a place to go at here in Washington!