We took a trip with our three kids in early May that included Arenal, Monteverde, and Tamarindo. Our youngest is 4 and a big adventurer who really wanted to go zip lining. We couldn't find a company in Arenal to take her, but we did find one in Monteverde. If you want to take your 6 year old on a canopy tour, I would recommend looking 100% Adventura up and adding Monteverde to your vacation. We thought it was really beautiful, filled with amazing wildlife and quite a bit to do.
Paolo Verde is another national park that is in Guanacaste and near where you'll be traveling. They offer boat tours where you can see monkeys and alligators and lots of other things. We had a really great bilingual guide that I can recommend if you are interested.
There are a few sailing catamarans in Tamarindo and a few others that leave from nearby Playa Flamingo. That was one of our trips of our vacation. There are also surfing lessons, stand up paddle boards, etc. Have fun! And, cheers to well traveled youngsters.

"We will have a car, but still have about 2 weeks left up the coast of Guanacaste, and time for a jaunt into Nicaragua."

Remember that a Costa Rican rental car cannot leave the country.

Thank you Jeff - I did know that- so on that subject, what would be your recommendation for going across the border, whether it be to San Juan del Sur, or to Granada... take a bus to the border, and walk across, or do a tour bus and go across that way, or a car rental that does drop off in CR and pickup in Nicaragua?

Hi,

When you cross the border you can easily take a taxi and in 30 mins be in SJDS - beaches / 2 hours Granada - colonial city, rest & bars / 45 mins the boat to Ometepe....Ometepe is nice for the nature - hiking, kayaking and horses....you have to be outdoorsy though.......happy travels! Sheri