This is definitely something I want to check out since I've only once been on an inner tube once when I was 12 and in my back yard! These adventures go through open canals and sugar plantations & let's face it, you're in Kauai & can't go wrong. Type A people can add the zipline.
Rent an SUP (stand up paddle board) for the week at Boss Frog's and pay $150. OR, you can rent it for 3 hours at the Mauna Kea, and pay $150. Oh, and Mauna Kea won't rent it to you until 8AM, and not if the beach break is over 2 feet (too dangerous, apparently).