I've done, and recommend:

• Icicle Creek
• Leavenworth -> Plain -> Coles Corner -> Leavenworth
• Blewett and Swauk Passes, as a loop (last chance 'til November!)
• Leavenworth (or thereabouts) to Steven's Pass (you could take SR-207 instead of US-2 if you can handle dirt roads?)

I'd like to ride:

• Winthrop to Diablo Lake (65 miles, ~5,000 feet elevation gain)
• Lake Roosevelt

Not a complete list, but good for starters. What kind of mileage and elevation are you looking for? I've seen maps of what looked like good loops from Winthrop and Omak, but don't have them handy.