My wife and I did a very nice overnight this past week in The Park. Hellroaring Creek, Yellowstone River trail, through Gardiner, then back on the Rescue Creek trail. A highly recommended overnight trip.
Many photos and some indulgent (boring) video can be found here: http://bedrockandparadox.blogspot.com/2009/07/best-idea.html
It was a pleasant time of year to do it. Bugs weren't too bad, days were nice, night was very mild. The 1Y1 backcountry site is highly, highly recommended.
Already planning more trips.