We’ve been! We really enjoyed our trip, loved the culture and the people were very friendly. In San Juan you do not need a car, but you will need one if you want to go to El Yunque (rain forest) for hiking. We really enjoyed the visitors center at El Yunque, and did an easy hike to a swimming hole; that’s all we had time for but well worth the trip. Do some research ahead of time to plan how to spend your time there.
We loved Old San Juan but you only need a day for it (although we stayed in OSJ for 2 nights at a B&B and would go back and do the same again.) Easy Uber to get there from anywhere in San Juan, and the rest is all walkable, assuming nobody has mobility issues. Check the cruise schedules to see which ships are in port on each day of your trip, and go on a day when the fewest ships/passengers are scheduled. Trying to get seating at a restaurant in OSJ can be tough when big ships are docked, particularly lunchtime. Plan to eat off hours or have a dinner reservation.
Loquillo is a fun beach town to visit if you want to experience local culture. Very low key with a surfer vibe. We actually stayed there for 2 nights and did an ecological kayak tour with a local guide, which we really enjoyed. (Did this instead of the bio bay tour which is popular, but has really mixed reviews.) There is a fun bar across from the beach called Boardriders, and also many choices for eating at the Loquillo Kiosks. But if you want resort feel or fancy restaurants, Loquillo is not your place.
As mentioned above, you might want to consider going to Vieques or Culebra. We didn’t have time but would probably do if we went back again.