Did you ever swim in a natural pool that is on an island swimming in another lake? Ojo de Agua is a great place to do exactly this in the middle of the forest.
Getting there is relatively easy. You can drive up to the parking with your car / motorbike or have the bus drop you off at the main road and walk the last mile.
You will need to pay 3$ per person at the entrance. Right afterwards you can dive into the pool, prove your balance on the slackline that spans over the pool or swing with the Tarzan rope into the water.
I suggest to get here in the morning as it can get crowded in the afternoon. Getting thirsty? Try out their cocount-rum-mix!