Hell, on the off chance.... I'll bite.
If you gave me my choice, I'd head straight to Montana and spend a full week and a half of fishing the Bitterroot, Rock Creek, Clark's Fork, the Blackfoot, and then head down to the Big Hole, Poindexter Slough, and the Ruby River. If you're looking for small stream fishing, try the North Fork of the Blackfoot and the West Fork of the Bitterroot.
If you're on the coast, and you want to steelhead fish, try the North Fork of the Stilliguamish River, it's a great little river. The Skykomish is a popular spot for steelhead, but I don't really fish it much so I'm not the person to ask about that. The Skagit is a massive river that I hear has quite a few fish in it, but it's too big for me. The Sauk get's good runs of steelhead but the water is crystal clear and the fishing can be very technical, it can be a difficult stream unless you know the holes.
On your way across I-90 heading towards Montana, I would suggest stopping in at the Yakima River and fishing for a day or two in the canyon. The canyon has great wading access, as almost the entire section is owned by the BLM, and there are lots of fish per mile. During that time of year, however, the water flows will be pretty significant due to them releasing water from the dam for agricultural purposes. Boat fishing is the best during this time, but there can be decent bank angling if you can fish it during a non-weekend. For some spots, send me a PM or talk to one of the guys at the Evening Hatch in Ellensburg.
Rocky Ford is East of the Yakima River about an hour and a half, located in the general area of Ephrata and northwest of Moses Lake. It's a very popular place, and can get very crowded, I would suggest staying away from it during the weekends if you can. It's a spring creek that's planted with large triploid rainbow trout and some broodstock fish, they read gargantuan size but they have become so accustomed to people that it can be a little like fishing in a fish bowl.
When you pass through the Idaho panhandle, you can check out the Couer De'Laine (spelling?) river, which I hear has a few fish. Still, I would probably do the same thing that most people do and drive right through the panhandle to Montana.
Best of luck and send photos!
worldanglr
http://www.worldanglr.com/
Calling Fly Fishing a hobby is like calling Brain Surgery a job.
-Paul Schullery
