Who knows what alpine lakes(particularly in Alpine lake wilderness area) are accessible by hiking and not frozen now?
I was on Surprise lake in mid-June (Surprise creek trail off Stevens pass HWY)
and it was still frozen

The twin lakes that are up off of the Mount Baker hwy are fun, but you can drive UP to them. Need a 4x4. It is about 40 miles outside of Bellingham. I can get you better directions if you're interested. Andy

Trek on up to Surprise Lake or Deception Lakes off of Highway 2 out of Scenic. The water is clear and you can watch those brookies all the way up to the fly. I'm sure many of the other lakes in the area also hold brookies.

Hyas Lake is supposed to be a good brookie lake as are Lost and Mirror.....

Hyas is up past Salmon La Sac and Lost and Mirror are sort of on the SE side of Keechelus... Hyas is about a 2 mile flat hike... Lost is right by the road and Mirror is a short hike.. (less than 2miles)... I've never fished any of them but one of my sources says that there are some good sized brookies in Lost, but you gotta work your way over to the far end of the lake...

MicroBrew.
I hiked this trail you're talking about. But not in suitable time. It was crazy idea to carry fly rod 5 miles to steep grade to find the lake frozen :-)
Now i'm gathering any useful information before to go somewhere

Spectacle up out of Salmon la Sac has loads of brookies, Pugsley off the edge of Lake Dorothy has them and also Mowich on the north end of Mt Rainer has them and you can drive to Mowich.

Also Leech lake has them but you have to fly fish there.

If you check out the WDFW website, somewhere on there is an "alpine" page with info on what's been stocked. Also there's a Washington state fishing guide out at most sporting good stores.

If you want more info on the subjects of "hiking" and "fishing" go to a Hi-Laker club meeting next month. You'll find lots of people there with similar interests and probably a hiking partner with more knowledge of alpine lakes.... and they'll be willing to share.

Awaken - I should have mentioned that it is not a day hike. I spent 4 days on the trip late in the summer and we had the lakes to ourselves. Caught many brookies up to 12" - but most were smaller. I have also taken the chair at Crystal up to the top and walked down to some of the small lakes to hit brookies. You can do the same at Snoqualmie and get to some small lakes off the logging roads. There are cutthoart in some of the lakes.

You might want to try Tye Lake near Stevens Pass. It's about a 2.5 mile hike North of the pass off of Skyline Ridge. Last time I fished there the brookies were starving and you could have caught hundreds in a day.