from Fortuna/Arenal area you can go to El Tanque and catch the bus to Los Chiles there, but it will be nearly always full (means you have to stand), so you might consider to invest an extra-hour or two and go via Ciudad Quesada.
On the Riverbank of the Rio Frio you can see sometimes Howler Monkeys and different birds. There´s a great path that goes along the bank, heading north from the dock. You´ll see lots of locals fishing, which is a great way to spend a day.
If it´s a weekend, you can probably catch an afternoon mejenga (soccer match) in the Central Plaza.
Soda Andrea (Donde Comer Es Un Placer), 100m oeste y 50m sur de la parada de buses (100m west and 50m south from the bus station), ☎ 24711107 / 87056878. 7:00am to 10:00pm. Local, family-run business, with menu in English and Spanish. Offers traditional foods of Costa Rica, specializing in rice, meat and fast foods. Totally natural, home-made fruit juices and smoothies. Well known throughout the community for their good service and good food. Ask for the owner, Andrea -- she´s a sweetheart. With plans for a small hotel next door. / Empresa Familiar.Ofrecemos la carta en ingles y en español. Les ofrecemos comidas típicas de Costa Rica, con especialidad en arroces, carnes y comidas rapidas. Bebidas totalmemte naturales preparadas en casa. Muy conocida por toda la comunidad por su buena atencion y su rica comida. Con proyecto de un pequeño hotel a la par.1200 colones to 3300 colones ($2.20 to $6.80). edit
Restaurant Heliconia, (next to Hotel Tulipan). open from 6 a.m., try the sea bass (local fish)! also organises morning tours on Rio Frio without the crowds from Fortunaedit