check out http://www.discounthawaiicarrental.com for the car rental--they might be able to do better for you. Though Christmas time is high season.

My Kauai "must-sees":

Hanalei Bay
Ke'e Beach
Lumahai Beach
Waimea Canyon
the Hindu Temple (tour must be booked in advance)

Sorry, but my crystal ball's not working for the weather. It could be anything. The three weeks I spent this past Jan/Feb were mostly gorgeous weather. The previous year, the same three weeks were mostly rainy.

Re: the car ...
Here's an excerpt from my most recent Kauai trip report. If you click on my name you'll be able to see my many Kauai reports--with restaurant recommendations, etc ...

I’d rented a car through Priceline and the agency was National. FYI, when I began checking, Hotwire prices were around $23/day (plus taxes and fees) for a compact car. Ouch! It added up to more than $850 for 24 days. On previous trips I’d had bids accepted at $13/day, but this time kept getting turned down at $14. I kept checking the websites and one day, I could hardly believe my eyes: $7.99/day at Hotwire. I grabbed it for a total of $463. A few days later that rate was gone—and back up over $20/day. The moral of this story is that it’s worth it to be patient and keep checking both with flights and cars.

When I arrived at the rental lot I was told there was no difference between the compact and mid-sized cars … and I could take my pick of anything on the lot! The woman who checked me out was quite surprised when I opted for the compact car.

PS. At that time of year the whale watching should be great. I saw lots of them every day just from my patio at Sealodge (Princeville). But the Kilauea Lighthouse is usually an especially good viewing spot, as is Spouting Horn on the South Shore.

Limahuli Gardens in Haena is a powerful place located just below Makana Peak (Bali Hai). Native plants and taro fields and friendly gardeners dominate the landscape.