Here are some ideas for "accessible" activities ...
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visit the bio-bay on Vieques - East Island Excursions has a tour that departs from Fajardo around 5:30pm via power catamaran. Once on Vieques, they provide all transportation, feed you dinner, and take you into the bio-bay (where you can optionally swim) aboard an electric pontoon boat. When all is said and done, they get you back to Fajardo around 11pm.
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go ziplining - La Marquesa Canopy Tours may be able to offer a night-time zip line tour. We've gone with them during the day. I think it would be a blast in the dark!
* there are some opportunities for
evening entertainment in and around Old San Juan. Most tend to center around Flamenco music and dancing.
* you might also consider contacting East Island Excursions about their sunset / moonlight cruises.
* Legends of PR offers a
guided night-time walking tour of Old San Juan that's interesting. My parents (79 & 81 years old) were able to keep up with the tour with no problem.
Hope this gets you started!