One of my favorites loops (more like a lollipop in shape) is in Olympic NP. From the town of Brinnon (on Hood Canal/east side of the park):
Follow the West Fork of the Dosewallips River up to Honeymoon Meadows, over La Crosse Pass, follow the Duckabush River up to Marmot and Hart Lake, over O'Neill Pass, over Anderson Pass and back.
This was a 5 night trip (4 nights backcountry, one at a campground b/c we got there in the late afternoon). On a map of the park, this loop looks laughably small but really shows how BIG the place is.
This is a nice loop for those in Seattle or Portland b/c of the east-side start/finish. Getting around to the Hoh Forest side takes a long time.