If you want a day trip away from the sun, then you can venture off to the center of the island and visit the world's largest radio telescope and a cave in the center of Puerto Rico, Arecibo.
The 130km drive takes around 2 hours from San Juan and 2 hours and a half from Rio Grande, depending on traffic. It is a lovely drive along Puerto Rico's mountainous system with some great coastal views as well.
I recommend visiting the Observatory beforehand not to be rushed as it closes earlier than the cave. At certain times they have a VIP tour which allows a visit into the control room and along more of the premises for an extra cost. Their opening days vary so check beforehand, but General admission costs $12.
If you have an extra day in the area, you can visit the Tanama River and even do some adventurous tours such as rafting and more.
NOTE: A COUPLE OF YEARS AFTER HURRICANE MARIA THE RADIOTELESCOPE OF ARECIBO CLOSED DOWN BECAUSE OF INSUFFICIENT FUNDS TO REPAIR THE DAMAGE CASED BY THE HURRICANE.