My experience is that water conditions are the key... when the temperatures, oxygenation, and clarity demand it the fish go into their summer behaviors. Off the cuff, I find summer fish in three kinds of water:
a) holding pools - Horseshoe in Palmer Kanasket on the upper Green. Loaded with summer runs by now, worth a look. Reiter Ponds / Sky. Fortson / Stilly. Lower Calawah. Fifth Bridge / SolDuc.
b) whitewater - riffles, rapids at heads of pools, turbulent tailouts, long bouldery runs, etc. Falls / Snoqualmie. High Bridge / Sky. Palmer above hatchery / Green.
c) classic - rock gardens, trenches, well formed tailouts, runs. Blue Creek / Cowlitz. Apple Orchard tailout / Elwha. Flaming Geyser Bridge pool / Green. Picnic / Stilly. Confluence / Hoh.
Not to mention other types of water that are in-between or a mixture of the above.
Classic water is the most satisfying to fish, kind of like winter water but much more trouty. Not like winter water in that things like stumps, cut banks, and all that structure really matters in summer. I recall a fallen tree where I hooked a ten pound chromer off my end, and Brian rolled a fish on a waking fly twice off the other end. My fish leapt four feet from the water and shook the hook. Brian's fish came up for his fly repeatedly but never connected fully. Talk about frustration!
As water gets depleted of O2 and warm, some fish tend to hang beneath rapids or in the heads of pools where water tumbles from the pool above it. If you hit Reiter each morning at dawn with a muddler style fly, concentrating on the rapids at the heads of pools, you could have a dozen dryline steelhead in no time. If you short-line muddlers in the rocky rapids below Snoqualmie falls for the first two hours of daylight through the summer, you will be in for some explosive summer run action over the summer. I've caught summer fish up to 17 pounds by standing upstream and longlining sinktips surprisingly shallow rapids, or gurgling a fly in the wakes of rocks in the rapids on a dry line.
Holding pools are frustrating in that there are so many fish to see but so few bite. On the other hand, they do bite once in a while and offer a great challenge to tempt. My best luck has been to target them at the earliest possible time of day and in the last hour of daylight with super-stealthy leaders and subtle flies like sparse dark speys or caddis patterns. I enjoy working these fish because of the challenge. There are always a couple of fish in the above two kinds of water in the vicinity of a holding pool as well.
They're all great!