I could not believe it when I went to the visitor center outside of Castle Rock (on SR 504) and realized it was the 30 year anniversary of the eruption of Mt. St. Helens. My wife lived in Idaho at the time, so she remembers the ash in the sky. It was interesting visiting the area. We found a good launching place for our visit at the Toutle River RV Resort in Castle Rock, Washington. The park was 3 miles north of the SR 504 exit.
On SR 504 (the Spirit Lake Memorial Highway), you head East from Castle Rock. After 5 miles you arrive at the Mount St. Helens National Volcanic Monument visitor center (Silver Lake Visitor Center) where you can see an excellent 13-minute video about the eruption as well as a number of informative displays about the volcano and surrounding area. I highly recommend taking the time to explore the visitor center and take in the video (when we were there they played the video every 30 minutes).
As you continue on the remainder of the 54-mile trek to the Johnson Ridge Observatory, where you are at 4200 feet and can see the crater across the North Fork Toutle River valley (5 miles from the mountain), you pass a few other visitor centers—Hoffstadt Bluffs (location of the Fire Mountain Grill where you can eat while viewing the mountain, and an old news car that was abandoned in the blast zone; not to mention helicopter tours), The Forest Learning Center (where you can see details about the devastation and the reforestation of the area—build by Weherhaueser Company who lost a significant amount of trees from their tree farm; it is fun to see the signs letting you know when the trees along the road were planted). You also pass through Toutle (location of the 19-Mile House Restaurant; where they serve very good burgers and homemade cobbler) and Kid Valley (where you find the buried A-Frame and the North Fork Survivors Gift Shop, with a very large statue of Bigfoot).
The drive is only 54 miles, but you should plan on at least 2 hours to get there because you will want to stop at the many viewing areas along the way. The day we went was cloudy, so we could not see the mountain. We went partway the next day when there were less clouds and realized that we had missed some great views; not realizing that the mountain was right behind the clouds.
When we arrived at the observatory the low clouds were moving quickly, so we waited until we could see into the crater—the north side of the mountain is the side that blew away, so you can see into the crater and the new dome that is forming since the volcano is still active. The picture is the best we could do on that day, but I wanted to share it anyway.
While the trip to Mt. St. Helens seems to be the most popular destination, there seems to be plenty more to do in the area (visit the Cowlitz County Tourism website for more information).
There are several things you should do prior to your visit to see the mountain. First, you should visit the US Forest Service website. This provides some excellent information (such as all the locations where you cannot take your pets) as well as a live view of the mountain (Volcano Cam). The cameras give you an idea of what you will see when you arrive. Of course, remember that it will take you 2 hours to get there, so the weather could change. It is definitely a day trip worth taking.