You will undoubtedly get many responses from riders intimately familiar with recommended destinations all along that route, so I'll just throw in some basic advice: First, go south to San Diego so you can ride east on I-8 over the mountains. You might be surprised how nasty the weather can be just a bit farther north that time of year.
Other than that, I highly recommend going south along the Rio Grande River through the Big Bend country, from Presidio, TX eastward. It looks as though you will have a lot of time to enjoy the ride, with lots of side trips all the way across the states. Have fun, be safe, and take lots of photos. We'll expect a complete ride report at the end.