As of now, I'm thinking of wrapping up the coast, plus venturing inland (and uphill) to see the telescope at Arecibo. I'm still considering visiting Caguas, the small city in the central valley. However, it still seems like quite a trek for a destination that does not appeal that much to me, so I'm leaning more towards not going. There are also numerous small towns in the hills throughout the interior of Puerto Rico. Again, they seem quite difficult to get to, and not that appealing to me. I think I would rather just move on to the coasts of other Caribbean countries.
After Puerto Rico, I plan to go to Hispaniola, first seeing Republica Dominicana, then Haiti. After that, I have more plans, but the exact route depends to some extent on transportation.