I travel out there for work (don't be jealous, it's fly out, spend 14 hr days in a concrete box, fly home).
But in the limited amount of free time I've had, here's my take on Kauai:
There are two Kauai, east side (nice, tropical) and west side (arid, semi-desert). Most of the main stuff is in Lihue; shops, Costco, etc. Best thing in Lihue is Duke's restaurant. MUST DO! And yes, you want to split the Hula Pie. I don't care how full you are...
Poipu is where I typically stay. Nice, not quite as expensive as the north shore.
Must see:
Drive up Waimea Canyon. You will pass through five climate zones, which is pretty unique. Be sure to plan extra time and stop at the coffee plantation on the way there or back. Don't buy the coffee, tho, get it at Walmart for half. Same with any trinkets, macadamia nuts, etc.
If you want to see the Jurassic Park part of the island, need to plan a helicopter ride. In fact, this is probably the best money you can spend on Kauai. Buy the LONG tour.
Have fun!