Caldera itself is really just an industrial port. There isn't much very close to it, so a shore excursion is recommended.
When we last visited we managed to find a private tour co. which met us at the pier with our own van/guide, and we had a wonderful day touring, but it wasnt easy to find.
We will be in Puerto Caldera in Oct. Have found a really good website for JD Watersports, www.centralamerica.com/cr/tours/jd.htm.
We have booked his River Cruise and trip to Sarchi for the day. Very good price and sounds like a great trip. We choose the ships tours in Cartagena, for safety reasons.