One of your first stops should be West Kauai Technology and Visitor Center. Here, you'll be able to orient yourself and get lots of useful information before you begin exploring.
You're spoiled for sightseeing choice when visiting Waimea. West Kauai Technology and Visitor Center is a good place to start here, but there are numerous other worthy spots to consider. Kekaha Beach Park, Waimea Bay Beach, and Russian Fort Elizabeth State Historical Park are close to the tourist information center and will round out your day nicely.