This was one of my favorite day trips that I took while in Puerto Rico. I did it on a Saturday, so make sure you check schedules/availability if you do it any other day. I am going give you an itinerary largely in pictures, since I am too lazy to write a detailed description. Plus, pictures are worth a thousand words. :)
Step 1: Rent a car. May I suggest Enterprise Car Rental? They were really amazing, and offer convenient pick up.
Step 2: Call the folks at Tropical Trail Rides and make reservations for the sunset tour at 4pm. (787) 872-9256
Step 3: Plan on arriving in Aguadilla at least an hour and a half to two hours before your scheduled ride so that you can have a drink or lunch. Better yet, get there even sooner so you can digest and relax on the beach!
Step 4: Drive to Aguadilla (directions found on Tropical Trail Rides website).
Step 5: Arrive and park your car at Tropical Trail Rides. Tell someone there that you are there for the 4:oo ride, and that you are going to walk down to the beach and have a drink beforehand (they won't mind).
Step 6: Follow the road down to Villa Montana, a stunningly beautiful resort on the beach in Aguadilla. Honestly, if I ever go back to Puerto Rico, and I have money, I will come to this resort, it is gorgeous. Plus Aguadilla is really one of those sleepy beach towns, a polar opposite from the hustle and bustle of San Juan, and I'd like to experience that aspect of Puerto Rico more.
Step 7: Stop by Villa Montana's beachside bar/restaurant, Eclipse. This place takes the cake for "most beautiful restaurant" I visited in Puerto Rico. I wish I had arrived in Aguadilla sooner, I would have loved to have lunch here. I did have a piña colada, of course.
Step 8: Go back to Tropical Trail Rides by 3:30 so you can fill out forms and get your horse. Here's me on my horse:
Step 9: Follow your guide through some greenery on your way to the beach.
Step 10: Horseback riding on the beach in Aguadilla. What else could you ask for?
Step 11: Stop so the horses can rest a while and you can explore caves, the beach, and some cool coral that has shells inside of it.
Step 12: Take shadow pictures of yourself on a horse (totally optional).
Step 13: After your ride, be sure to tip your guide, then head back in the direction of San Juan, stopping at Molina's Restaurant ( Carr. #2 Km 117.6, Bo. Ceiba Baja
Aguadilla, PR 00603) for a delicious seafood dinner. It's a little pricey, but definitely worth it, since you will be tired and hungry after horseback riding. Additionally, on Saturday night they had an amazing band playing (see video below). It was a perfect ending to a perfect day. P.S. I had the shrimp cocktail and the surf and turf, both were amazing and I even had leftovers. I also had a piña colada. Of course.