-- The South Rim is a can't miss photo op if the sky is clear, but it's a 13 mile roundtrip hike (though not too steep anywhere).
-- The drive along the Rio Grande between Big Bend and Presidio, Tx is really pretty in the late afternoon sun too when the mountains are painted orange. Of course, you want to be doing the opposite direction from Presidio to Big Bend at that time (nothing too scenic about looking straight into the sun, lol).
-- If you do the Window Trail (~4 miles roundtrip I think) you might be able to see some black bears. I have a friend who got some great bear pics on it and we had a black bear come down from that trail into the Chisos Basin campground when I stayed there before (I saw it walk back up that trail exiting the campground).
-- The escarpment on the Mexican side of the border just before Santa Elena Canyon is really pretty, as is the canyon itself. It can't be more than a mile and a half as a roundtrip hike through the canyon. I never have taken a boat through the canyon, but I imagine that's the way to see it (can't get too far on foot).
Edited: 12:18 pm, February 19, 2012