Fun City is on this side of Playa Dorada (meaning the Sosua side, east). About a 20 minute taxi ride from Sosua. The water park is just this side of the PP airport. About a 8 minute taxi ride. So I would think you could do both in one day, start with Fun City before it gets too hot............then return to the water park for the afternoon.
There is not much to do at Fun City IMHO. After you drive around the tracks a few times, that's pretty much it. The carts are fun but it doesn't take too long..........unless you plan to keep paying to go around and around....................