"The depot creek waterfall is amazing. Off hand I don't know of any other several hundred foot roaring waterfall/cataract that you literally hike through/next to."
+1 It is quite a sight, and also a good place to watch your step. I've seldom been on more slimy rocks. We used a hand line coming down. Left side is definitely where you want to be most of the way.
"Never been on the Cooper ridge. How is it? I take it that you have been there? Seems like a great weekend trip."
Copper Ridge is one of the routes I haven't done, but I got a blow by blow description from a couple of friends who did it as a weekend trip, and had a pretty good look at it on the way in to Mt Challenger. Excellent views but, for my money, Easy Ridge offers a lot more bang for the buck. It can also be done as a weekend trip, but 3 days would be better. That way you could drop down around Imperfect Impasse and climb up the other side to Perfect Pass without pushing too hard. From there you have gorgeous views of Whatcom Peak up close and personal, along with Mt Challenger and lots of other peaks I can't recall off the top of my head. Bring a map and compass and identify as many as you wish. It's a beautiful area. The route is best done later in the season, as you have to ford the Chilliwack River and, if it is running high, it would be pretty sketchy.
While I've got your attention, if you haven't already done the Bath Lakes High Route or The Buckindy-Snow King Traverse in The Glacier Peak Wilderness, you might want to put them on your to do list. They are both classic Cascade off trail routes, strenusous, beautiful, and remote. It sounds like they'd be right up your alley.
Elizabeth,
You've made a couple of excellent choices. The Pasayten will be far more enjoyable in September. I hope you have a great time!