You're really not going to find any spots on the west side of the mountains to keep fish from streams. As stated above, the streams are generally more sterile. A lot of these streams offer only a couple of fish per hole, if that. The sterility of the stream makes the fish that do survive, very willing to take a variety of offerings. This means that it is very easy to fish out a stream, or a section of stream. To keep fish from these places, though it is perfectly legal to do so, is unadvisable. The fish are small and skinny, too. Around highway 2, a lot of people camp and keep stream fish and the fish population suffers because of it.
There are sea run cutthroat in most rivers this time of year. Some people will catch and eat them because they run a little larger, but it is highly frowned upon. It's illegal to keep them in saltwater, but legal to keep them in fresh. Go figure. As for Coho, go for it if you want to hate yourself. Every time I go coho fishing, I see them surfacing all around me. I think before I get my line in the water that it's a sure thing. But I never catch any. And not many people really do. Most that are caught are foul-hooked. The ones that are fair hooked are often the result of foul-hooking them directly in the mouth when the line slides through (lining or flossing). There are some fisherman who have it down. It generally requires near-perfect conditions. Most of the people that consistently catch them fish out of a boat with gear and definitely have their game down.
Steelhead fishing is a lot of fun though. You won't catch many, though. You'll likely spend ten times more money fishing for them than paying for them in a grocery store. It's definitely nice to bring home a hatchery brat once in a while to prove to family and friends that you really do go fishing, and are not just a serial killer with the perfect alibi.
If you can stand a little bit of hiking, higher elevation lakes are a lot of fun to fly fish. You can generally keep fish from them with a free conscience, as almost all of them have been planted at some point in time and the fish are overpopulated anyway.