I'm certainly not the most qualified here to answer your question, but seeing that nobody else has responded in the last four months, maybe I can bring this thread to the top and encourage some more responses. You didn't mention if you were just wading or if you were hiring guides or bringing along something that floats, which would probably make a difference in some cases. Personally, I enjoy trying to solve the riddle myself and tend to be kind of minimalist on my trips, living out of a 7x7 tent for 6/7 days and maybe one night in a motel to clean up, etc.
In "real life" (whatever that may be), I live in MI (SE part of the lower peninsula, parents and grandparents were Yoopers). I have hbeen fortunate to have many chances to travel out West for work in the past and have more recently been taking a couple months off over the summer to go on a self-directed trout safari. I've never swung for steelhead in the West, but have fished for trout with several techniques. Here are some places I've swung flies that I wouldn't mind going back to--some I've used a 12.5 ft travel spey, 11 ft switch rod, as well as a SH rod with either a Wulff Ambush line or a line I cobbled together myself:
Bow River downstream of Calgary Alberta: In August (and perhaps July) they have hoppers which can make for fun dry fly fishing. The highlight was definitely what the locals call "skid-*****ing", skating a stonefly imitation across on the surface. There were some nice sized fish too. Had a blast and bent out a couple hooks.
Lower Madison River near Bozeman/Ennis, MT: By this, I mean below Ennis Reservoir from Beartrap Canyon to Greycliff Campground or so. This is a tailwater section and gets too warm to fish in the summer. I did well swinging crayfish and sculpin patterns.
Big Horn River downstream of Ft Smith, MT: This is a famous tailwater and can be crowded at times. You can wade downstream from the dam area. I had the place to myself in November (but it was cold and windy).
Green River below Fontanelle Resevoir, WY: Found out about the campground here from Larry and stopped by several years ago. Everyone was fishing midges but I had seen caddis in the air, so I swung up some sz 12-14 chartreuse pupae patterns and was having a ball until I had to haul a** back to my tent to keep it from getting blown away in a sudden summer thunderstorm.
Missouri River near Craig, MT. Okay, I haven't swung flies here, but it's on the list. I need to do some research on access, etc as I plan out this summer's adventure.
Some other places to consider:
Rock Creek, near Clinton, MT is a favorite of mine. It's reasonably close to town, there is camping right along the river if you don't need all the comforts of home, and a motel near a fly shop (Rock Creek Merc-- good people) right at the freeway exit.
Depending on your timing, you may have a good chance to fish for some of the aggressive browns headed upstream in the fall. I know guys throw 2 handers up on the upper Madison in MT at that time of year.
Have a great trip. I hope this is of some help to you, and I also hope that some others will chime in. I'll likely be on the road fishing this year from late June to Labor Day or so..time to get some planning and organizing done.