The Ultimate Kauai guidebook is worth the money, although if you stay at a private home/condo, they will likely have that on the bookshelf.
Do some type of tour right off the bat, it will give you a chance to see all the cool touristy places and a few other places people miss.

When I come to Kauai, I usually do a drive of the island (from Ke'e to Polihale Beach) to see what might have changed, then divide the island up for the remaining days. The drive end to end isn't a long one (80 miles), but expect it to take 2 1/2 hours minimum, as there always seems to have construction.

If you want to know the favorite local spots, get to know a local (bring beer or Malasada donuts) at one of the beaches (my favorite beach to meet a local was off Niu St in Kapaa) in the evening. You will be more likely to see locals enjoying the weekends at Anahola Beach; that is one of my favorite beaches. If you are Christian, visit one of the local churches on Sunday, lots of local tips will be given out and you might even get invited to lunch at someone's house.

Been there 4 times, twice in 2013. Stayed in condos and houses, never a hotel. And it truly is Paradise!