If you are in Pedasi and you want to go to a river, you can go swimming in Rio Oria, which is about 25 minutes from downtown Pedasi. You can also go horseback riding over there as well and perhaps, hiking, too. You can go there through a tour company and it might cost you a bit.
I’m not sure of any lakes or waterfalls here in Pedasi. There might be some waterfalls on Cañas Island (about an hour away by car) during the rainy season, but it is nothing big or famous.