Here is a short description of a hike I did in Aug. 2008 and some photos.
3 of us started at the Howard Eaton trail head near Old Faithful. Followed the Bechler River Trail all the way to Bechler Meadows. Took a trail through the meadows over to Union falls then finished up at the Grassy Lakes trailhead. Hiked 56 miles in one week (mostly down hill). Nice moderate hike. Didn't come across any bears or sign. Only saw one lone bull moose and a couple deer. No mosquitoes late in august.
Thermal region up from the lower Bechler Ford is awesome! So was the scout pool by Union Falls. Bechler Meadows has been my favorite section of Yellowstone for the last 10 years. Probably the best kept secret in Yellowstone. If you have time I recommend an extra day to check out Dunanda Falls as well. I've been before so this trip I added Union Falls. It would have been nice to hit both though. The only bad part of the trip is the car shuttle. We dropped one off at the Grassy Lake TH for the exit. Made for a long day of hiking followed by a drive to Old Faithful to pick up the second vehicle, then home.