You should have plenty of time to do both. The ruins at San Gervasio are moderate...I and you don't really need a tour...we wandered around on our own. The largest ruin is way in the back...hike on the trail thru the woods about 10 min....a temple to the fertility goddess Ixchel. Lots of iguanas sunning themselves. You probably won't need more than 1-2 hrs there and from the car rental place the drive is no more than 15-20 min. Afterwards, you can continue on toward the East side of the island where there is a great place for an early and affordable lunch...Coconuts...amazing views!!!! Travel on toward San Miguel (one major road around the island) and stop at one of the beach clubs along the way (Paradise, Nachi Cocum etc) for a little beach time before dropping the car off. Have a great time!!!