The 30 year-old Mt. St. Helens Visitor Center at milepost 5 is also open if you have time on the way down, I would save it for last ($3/person). Not sure what to see ?  Its easy - stop at M.P. 33 at the Mount St. Helens Forest Learning Center (Free Admission!) for great exhibits and cool shopping - and a great Mt. St. Helens adventure for the whole family. For the highlight of your Mount St. Helens Vacation adventure go to the road end (52 miles from I-5) at Johnston Ridge Observatory for the photo-op of the day and watch the short movie there. Its $8/person to go in the visitor center but believe  me - it's worth it and your contribution helps maintain the Monument. Take a Virtual Drive up the Highway right now starting at the Mount St. Helens Forest Learning Center.