We started the day very early, as it was going to be a long one. Most people go from Arenal to Monteverde, or vice versa, but we had decided to take a detour to hike in Rio Celeste. The drive from La Fortuna (Arenal volcano) to Rio Celeste is ~3 hours (don’t believe Google), on a very bad road, through San Rafael de Guatuso. It gets even worse if you continue further, towards Monteverde, so it should definitely only be attempted with a 4WD.
At the Rio Celeste National Park entrance, the rangers offer you a guide and claim that it is necessary… But you can totally do it on your own, just print a map of the trails that you can find online, or ask them to draw it for you. The hike took us 4-5 hrs, including going down and back up for the two waterfall viewpoints (would recommend only one, the one with stairs), and a quick lunch. The trail was very muddy and went up and down a lot, so the hike is more exhausting than one could think. But the views are so worth it. The river, which at times boils with volcanic activity, turns from orange into intense turquoise and explodes in a big waterfall, framed by lush green forest… spectacular!