It Goes Against the Grain, But...
I seldom advise people not to take in an attraction, especially a natural feature, that they find interesting, but in this case I might suggest that you at least reconsider going to Mount St. Helens. Except for the fact that it erupted within memory, most people would never have heard of this particular volcanic mountain. But that eruption was an event, a point in time that is now several decades in the past. There is very little evidence of it sill to be seen. Now understand that if I were in your shoes I would go, but then I'm a geologist and indeed Mount St. Helens erupted while I was getting my undergraduate degree and we studied it quite a bit. But the fact that it is an active volcano also means that it's not really worth it to the US Department of Agriculture's Forest Service (which runs the monument) to spend a lot of money on permanent infrastructure. There is at least a short ADA accessible path/viewing area at Coldwater Lake Recreation and that as well as a Visitors Center at Johnston Ridge Observatory, but as Dave noted, it's a long drive in to those sites from the main road and a long drive back out (100 miles round trip from I-5), for what will likely be little more than a photo op. If it's something you or your mom have you hearts set on, then it's eminently doable. But if you're just going because it's a place you've heard of, then your time would probably be better spent in the more fully developed Mount Rainier National Park, which also has the advantage of being a 'drive through' that does not require backtracking.
AZBuck
