There are a few viewpoints along the road leading from highway I-5 to Mount St. Helens, but the best (and closest to the volcano) is the Johnston Ridge Observatory.
The Johnston Ridge Observatory is at the end of State Route 504, and the visitor center overlooks not only the yawning crater of Mount St. Helens but also a huge swath of the blast zone. Exhibits walk visitors through what happened on the morning of May 18, 1980, as well as the scientific history that led up to that enormous eruption. There are trails that begin at the Observatory for day hikes, including a half-mile trail that offers great views into the crater.
Johnston Ridge is named for the volcanologist David Johnston, who was surveying changes in Mount St. Helens in May of 1980 when it finally erupted. He was never found.
Because of the elevation of Johnston Ridge, the Observatory is not open year-round. It typically opens in mid-May and stays open until late October, operating from 10am-6pm daily during that season. Inclement weather can alter the opening and closing dates, so find out if it's open before you set off.