Lake Arenal and its accompanying volcano of the same name are one of the natural gems in a country replete with preserved natural places. Approach the Laguna de Arenal from the north at San Rafael de Guatuso along Route 143. Make a visit to the town of Nuevo Arenal. When we arrived it was towards the end of the day so we found a place to camp at a disused boat-launch park.
From here you could take windy Route 142 east directly to Tabacon, enjoying views of the lake and vocano and then continue on towards San José. Or you could ride counter clockwise towards Piedras. Shortly after the town of Tejona, Route 142 veers south. Or you could continue along the according to Google unnamed road that follows zig-zaggedly along the shore all the way to El Castillo at the lake’s southeast edge.
After El Castillo, you can enter the Parque Nacional Volcán Arenal and then follow the La Fortuna-El Castillo Rd to La Fortuna.