Hi. You're gonna love Kauai.
First, Here's another thread with some great info:
http://tugbbs.com/forums/showthread.php?t=226781&highlight=taterhed+weather
Next, my opinions:
The airport and, more importantly, the Costco, are in Lihue. There are hiking and adventure tours available from most areas, but the quality places may be north-only or south-only depending on time of year or company. You can hike amazing trails from Northshore and amazing canyons from Waimea. But, these two parks are 1.5 hours drive time from each other (All dates times are approximate of course) Generally, the Northshore is calm and swimmable from May to Oct. Southshore is nice from Oct-May. Your mileage might vary. Kauai contains desert-like canyons and rain forests. Amazing.
Now, not to p.o anybody
but, there are several 4-5* resorts: Westin Princeville, Marriott Lagoons and Marriott Kauai Beach Club in Lihue, Marriott Waiohai in Poipu. These all have/share fancy pools and hotel-timeshare characteristics. Poipu beach is the best beach IMHO. Marriott KBC is probably the best pool (but limited kitchen). Westin is probably the best resort/rooms.
There are many other wonderful resorts: Wyndham, Beachboy, Luwai, Point@Poipu, independents...just check the reviews. Every one of them has their own ups/downs from the reviews and pics etc... I've toured some and stayed in others. We bought Waiohai, so I'm biased.
Probably your best bet: decide what you must do (canyon? snorkel? whales?) and then plan your visit. Or, get your exchange and then plan based on driving. I'd avoid lots of planned driving from north to south/vice versa. Lihue;Kapaa gets busy in the afternoon with traffic during high seasons.
my personal rec'd: Jan-Mar, stay in Poipu (Waiohai, Luwai, Point). Know that some views at these resorts are parking lot or sand dune or building. Nice beach, nice hike, nice food, low traffic. See the Canyon. Hike the south end of Napali, walk the heritage trail at Poipu. Love hiking? (mod) go to Hanakapiai Trail at far northwest end of road. Best poke you'll have is at Koloa fish market.
Have a great trip...you really can't go wrong.