My DH and I have been to Kauai three times (once for our honeymoon, once for vacation, and once for our babymoon). We both love it - my DH is very active (hiking, snorkeling, swimming, etc.) where I would rather lay on a beach, and Kauai is great for both. And September is our favorite time to go!
We prefer the South Shore because it's warmer with less rain, but the North Shore is very lush and beautiful, so you really can't go wrong. We went on a Na'pali day cruise and went snorkeling off the shore, a guided day hike with kayaking and ziplining which was so fun and met a lot of couples from around the world on their honeymoons, plus lots of hiking and snorkeling on our own. We rented a condo through VRBO.com at the Kiahuna Plantation on all three visits, mostly beacuse I wanted a kitchen so I wasn't forced to eat out for all meals.
I also recommend the Ultimate Kauai Guidebook for ideas on food and activities:
Have great time!