It's a good workout kayaking to the falls. On the way there you are kayaking against the current of the river coming down, and on the way back you are kayaking against the ocean current coming up the river. The falls are 200 feet high and you can swim in the pond below and stand in the falls. It's ice cold but well worth the experience.
Kauai has the only navigatable rivers in all of Hawaii. It's one of our favorite activities to do year after year, although it has become quite popular and can get crowded, so go early, at around 8am. It will take you about four hours give or take. The location is by Kappa Town, a fun place to have lunch after.