Hello Rosemary! Sounds like a fabulous trip.
If you are interested in volcanoes - as I am - there are a great many in the areas you will be travelling through. Most (like Mt. Hood) are massive - and you might not even know that they are volcanoes, if you only see them at a distance. The area around Mt. St. Helens can be "iffy" to find reservations - St. Helens is a major tourist destination - I suggest making reservations at Harry Gardner State Park in Toutle, Washington. It is about 25 miles from the Johnston Ridge Observatory (the observatory is not to be missed) at St. Helens. I stayed there two years ago, and (for my purposes) it was very nice. Sites were very
widely spaced, water and electric (I think - be sure to check) and a dump station, and vault toilets:
https://www.co.cowlitz.wa.us/facilities/facility/details/2-Harry-Gardner-Park-2The park is right on the South Fork of the Toutle river, and the park was completely wiped out by lahars (mud flows) from the volcano during the eruption of St. Helens (although the volcano is more than thirty miles away!), and the park was only finally rebuilt and re-opened in the last couple of years. Remnants of the lahar flows are still there and still evident, especially river-side, if one knows what one is looking at.
Here is a short article about a couple that was caught in the lahar - it was at Harry Gardner that they were finally able to escape the lahar, as it washed in, burying the the park in feet of mud from the volcano. They were very nearly killed:
https://www.pdxmonthly.com/articles/2017/10/12/survivors-of-the-mount-st-helens-eruption-tell-their-storyIf you are interested in volcanoes, my all-time favorite volcano book is "Fire Mountains of the West, the Cascade and Mono Lake Volcanoes" by Stephen L. Harris:
https://www.amazon.com/Fire-Mountains-W ... way&sr=8-1Although it is non-fiction, and a bit technical, it somehow also reads like a combination of a travel/adventure book. But maybe that is just me!!
Also keep in mind (sorry if you already know this!) but Oregon and Washington west of the Cascades (of which St. Helens is but one volcano of many in the Cascades) is generally cool-ish, lush and green in summer, on into the autumn months.
East of the Cascades, Washington and Oregon are desert and can be
very hot in the summer and autumn months (think 90's on into the 100's). I was there at Harry Gardner in late September, and temps were in the upper 90's - and this is still
west of the Cascades!! There is
no shade at the Johnston Ridge Observatory at St. Helens (all of the amazing conifer forests that once surrounded the volcano were wiped away by the eruption, and have not grown back), pets are not allowed on the grounds or in the buildings, so not so good if one is travelling with pets.
For my money, (LOL - since I am so interested in volcanoes) I would also visit Newberry volcano, near Bend, Oregon (it would be a quick stop). It is a shield volcano (St. Helens is a stratovolcano) and has an amazing and very large obsidian flow (obsidian is volcanic glass) well worth viewing. Interestingly, the city of Bend is built on ancient flows from Newberry - and Newberry very likely will erupt again one day! This could happen fairly quickly - from the time St. Helens first indicated she might be "waking up" to the time of her major eruption, was only about 8 weeks!!
Any questions, please ask!!
Anne