re: Kauai- 10 days

The heritage trail at Shipwreck Beach is a nice, relatively easy hike that runs along the southeast coast. And a great place to swim when you are done with the hike. (You can't really swim along the Kalalau trail). As mentioned above drive as far up Waimea Canyon as you can and there are some cool trails.

If you're going to hike, bring some good hiking shoes and be careful. The island gets a lot of rain and footing can be treacherous in a lot of places. They don't do much trail maintenance or install any safety features because they don't like "disturbing nature". There were a few places where I said, "this isn't safe" and turned around.