If you are just doing a day out of San Juan, would either drive east to El Yunque National Forest with a stop at Luquillo Beach (nice place to take a walk, and there are tons of local food stands setup there). Or perhaps drive inland to see the Arecibo Observatory.

On my last trip I drove around the island for a few days heading west from San Juan (stayed a night Aguadilla), visited Rincon to see the surfers, then drove down to Cabo Rojo area (stayed another night) and then headed to Ponce for a few nights. It was ok - glad I did it - but the drives took longer than expected and while certain areas were nice (i.e. several of the beaches are beautiful), a lot of the drive was just Burger King after McDonalds after Burger King. Not that scenic. So I would not recommend going to the west end just as a day trip.