My favorite area in Washington State is the Glacier Peak area (and we have a lot of great areas). I haven't done the hike, but I can tell that a loop which included going over Spider Gap and Buck Creek Pass would be fantastic (everyone who has done it has said so). This would include Image Lake (one of the more photographed spots on the planet). Since the big storms have damaged a lot of the trails, this is the easiest way into the lake.
There is a lot of up and down (the Sierras are much kinder on your knees). Unlike a lot of areas, you don't need to worry about reserving campsites. I would recommend late July to early August. By then the snow will have mostly melted and the flowers should be in full bloom.
If you get on http://www.nwhikers.net, you can find plenty of people who rave about the hike and offer advice and tips (I'm sure some of the folks on this forum can do the same). Allow time to explore little peaks and side trails as well.
To give you an idea of the area, I googled for pictures and came up with this: http://bobrat.weblogger.com/stories/storyReader$1434 (I don't know the people). They went in late August, so they didn't have to worry about snow or bugs too much. If you go in early August, you'll have plenty of bugs, but wildflowers that will be superb.
