Visit the amazing cerulean blue river in Tenorio National park – a lovely area for walking. The color comes from sulfur and calcium carbonate in the water from the Tenorio Volcano. This makes a great day trip from Arenal (you can even soak in natural hot springs along the river but watch the temperature). The website link below does a better job of describing the area than the park website. Get there early – visitors are limited and there is no online ticket system.