The Boundary Trail to the Mount St Helens Lookout is a short 1 mile hike that starts and ends at the Johnston Ridge Observatory. The trail is mostly uphill on the way to the main observation point, but it's paved and accessible. You can hike as far or as short as you want since Mt St Helens is within plain view from the beginning of the trail.
From the observation points along this trail you can see for miles around where the mountain spewed its ash and covered most of the vegetation. Hiking within the blast zone requires a permit since the vegetation is slowly growing back and the park system does not want to disturb the land too much. The volcano is still technically active as well, so this may be another reason why they keep hikers and backpackers to a minimum on the trails within the blast zone.
If you plan to visit Mount St Helens, this is definitely the trail to stop by; however, the drive in takes a while. You'll be driving on WA-504 one-way for a while into the wilderness before you reach the Johnston Ridge Observatory, but it's well worth a trip.
Note: This road is seasonal and is likely closed in the winter, late Fall, and early Spring.