My wife and I pretty much did about 80% of your planned route (both south and north bound legs), in two separate trips, about 2-3 weeks ea: CA to Victoria, BC and from CA to MT. How much time are you planning on allotting for the trip? There's just way too much stuff along the way. We kinda played it "loose" and just had 2-3 big "must visit" spots and just stopping at anything interesting along the way.
- Since my wife has a beginner’s experience of driving a manual transmission, and to give me a break from driving I’d like her to drive on fairly flat and less windy roads. Are there any segments of the I-5, from Washington to California specifically, that are relatively easier or harder than other parts?
I-5 from WA to CA is pretty much a flat straight shot, very little to "worry about". I think she might have to down shift in a few spots when you start climbing and descending into the CA valley and then back up in the Grapevine.
- Are there any must see stops along my route that we could check out? We’ve been to Seattle and Portland but the stretch from Portland to California we haven’t seen yet.
If you have time to spare, I would actually recommend taking 101 instead if you want to do stuff along the coast, plus Redwood Nation Park is right along the way. If you must take I-5, Crater lake in OR is pretty cool and a small detour off I-5, same way with Lassen Volcanic Nation Park off Redding. There's pretty much nothing along the CA central valley, but I would recommend you stop by Harris Ranch in Coalinga, CA for a steak (you'll smell it before you see it).
- A couple of places I want to see while I’m down there are the sequoia national forest and the Bonneville salt flats. Any of you all been here and is it worth it?
Yes on both and well worth it. You can actually drive on the salt flats if you want to, its a very cool and surreal experience, but I wouldn't recommend it in our cars unless the salt is super dry and hard (very easy to get stuck and you'll get salt EVERYWHERE)
- From Salt Lake north, I’m not sure what route to take or whether there are spots worth seeing.
If you have time, I would venture further east into WY and definitely spend a few days in Yellowstone National Park.
Good luck and have fun!