Firstly, the park is enormous. You're not going to even come close to seeing all the great stuff it has to offer in 2.5 days. Just spend quality time in the areas you do visit, and plan subsequent visits to explore more new areas.
Secondly, don't confine yourself to a set list of places that offer the "best" views. Pretty much any place in the park offers great scenery if you take the time to explore it and enjoy it.
I'm often amazed at some people's photos - they're amazing views taken in sometimes "out of the way" spots.
Now that being said, some of the well known, iconic, and touristy places in the park include:
- Santa Elena Canyon: direct sunlight hits the cliffs at sunrise, and late afternoon light pours through the canyon from behind
- The Window (from inside The Basin): direct sunlight hits the Window just after sunrise, and sunset is pretty nice viewed through the notch
- Sotol Vista Overlook: big sky views good for sunset
- Boquillas Canyon (the overlook is just okay; I'd drive on further to the trailhead and hike into the canyon) - the canyon will probably be in heavy shade for both sunrise and set (someone can correct me on this?)
- Rio Grande Village nature trail and overlook - definitely a good place to see the sunset
- Ernst Tinaja - very good for late afternoon and sunset - a little more difficult to get to vs. these other places
If you want a good taste of the Chisos but do not want to hike a lot, then try the Lost Mine Trail. It's relatively short (2.4 miles one-way) and only ascends 900 or 1000 feet. The view from the top is spectacular. It's great for sunrise (have to hike up in the dark) and also nice in the late afternoon.