A two-hour drive north west of Arenal brings you to the sleepy Tenorio region, its volcano and national park of the same name much less-frequented than Arenal. The national park here is home to some superb (if challenging) hiking through dense rainforest trails and past thermal mud pots.  Star of the show is the Rio Celeste waterfall. The water’s unique reaction to the minerals coating the rocks on the river bed here turns the pool of water an impossible shade of turquoise.