I am a gym rat and usually go to one of the gyms in Sosua which has a good number and a wide choice.
Last week I found myself in Puerto Plata and decided to try the American Gym. This is an excellent operation. Its clean, well set up and inexpensive. 50 pesos for a day as a non member. The floor is rubber matted and spotless. The machines are top quality. They only have fans but enough to do a good job. Staff was friendly and helpful. For cardio persons they have good bicycles and spin classes. What I found especially appealing was the layout of the place. The free weights and the machines were placed by someone who knows their way around a gym and not just set here and there. One other appealing aspect is they have high ceilings so you do not feel you are working out in a dungeon. I have no connection to the gym and I am not being paid or offered any incentive. I have been to gyms all over the world and know my way around a dumb bell. This place is top flight. Every time I am in PP I will work out here.