Kauai is the honeymoon island, I can tell you if you want night life, there isn’t any, everything pretty much closes at 10pm and opens at 9am, very laid back and great for amourous activities. Try to stay towards Poipu beach, it’s sunnier and less rain than the north side. Septembers weather is really nice too. For a wedding location I must recommend Smith Family Ranch, they have a beautiful tropical garden there and they have one of the best luaus I’ve been to.
The beaches on Kauai are all beautiful. I’ve been to spouting horn, poipu beach, tunnle beach, all great places.
Kauai also has great adventures. Kauai has a ton of outdoor activities like Kauai ATV tours, picnicing and swimming at a waterfall (take the long tour, only with Kauai ATV, they are the best), or riding an innertube down the old sugar cane troughs in the hills, zip-lining, horse back riding on the beach, plus endless water sports, just so much to there.
Check this site out for ideas http://www.101thingstodo.com/kauai/
Also when you arrive in the airport, when you are in baggage claim, make sure to pick up all the brochures you can, great ideas and coupons.
check this site out http://www.kauaiislandweddings.com/Pages-New/Locations.html
This will give you links to affordable places to stay on Kauai under the Honeymoon Lodging tab
FYI: Don’t bother with the Fern Grotto location, they don’t let people get close anymore because of falling boulders. Also Hanalei Bay (the north side of the island) gets the most rain, but when it’s sunny it’s gorgeous.
You might look into staying in a time share where you can save money by cooking your own food, I always stay at the Waiohai Marriott Resort. Most I can’t offer any advice for Maui, I’ve never been there, but I do know that is a party island. Maybe get married on Maui and island hop to Kauai for a honeymoon adventure.