Medium level hike to access a lagoon.
This is a nice hike of around 4 hours for normal people. If you are in good shape you can do it in 3 hours. You can access Cerro Chato form two places. This page is about the entrance from Arenal Observatory Lodge & Spa. You need to pay 10 USD at the entrance but you can access the Cerro and also enjoy a Waterfall.