This is a great trail because of the many options it presents (not to mention the incredible scenery!). If you are really adventurous and the river is high enough, it would be possible to float down from Boxley and hike back up. The river doesn't get that high too often, though, so you may want to skip that short section of the trail and begin at Ponca, or Steel Creek, or any access you so please (and the trail from Boxley to Ponca is not used often, so it's typically overgrown). Beginning at Ponca you also have the option of making an unforgettable loop by taking the Old River Horse Trail in conjunction with the BRT. A word of warning: in the summer, when the multiple river crossings are most inviting, that horse trail can be a nasty, nasty thing. VERY overgrown and completely lined with ticks of every variety, not to mention snakes. But when the water level is low (as is typical in the summer) it is quite possible and extremely fun to use the bed of the river itself as your path for long stretches. My suggestion if you'd like to use the horse trail is to do it on a warm day from mid- to late fall.
Of course, with the multiple accesses along the trail, there are numerous options for shorter hikes and side-trips. I highly recommend a trip to Big Bluff & Hemmed-in-Hollow (just head left at the sign between Ponca & Kyles for a 2-mile, 900 vertical feet, trip down the the Old River trail, take a left on the ORT and follow the signs); and a trip up Indian Creek (I consider this the most impressive drainage on the Buffalo).
The BRT itself is FULL of exciting scenery. If I had to recommend one section of trail over another, I'd choose the section from Ponca to Kyles. The ORT is in the best shape between Hemmed-in-Hollow and Kyle's, and it's pretty decent from Kyle's to Erbie. Whatever trip suits you best can be found on the West half of the BRT, from half-day outings to weeks-long explorations.