Mt. St. Helens
Mt. St. Helens is a volcanic mountain about 50 miles north of Portland. It's notable for blowing its top (side, actually) on May 18, 1980. Helens is still active, including a 2004-2006 period of growth and small eruptions. On clear days, Helens is often visible from Portland.
The other way is to go to the east side on Forest Road 25. This route goes north from Sprint Lake to Randle, Washington. The road is very bumpy and narrow. It is not advised for large vehicles and RVs. The side route of Forest Road 99 gives visitors a great view of the crater and of Spirit Lake.
Both of these roads are closed in the winter and typically don't open until late June. Check the current road conditions for the status of SR504, FS25, and FS99 before going.