Kauai is awesome, great choice! Poipu is where we stayed and it was spectacular. A few recommendations from me:
For hiking, this trail is an absolute MUST
http://www.kauaiexplorer.com/hiking_...apiai_hike.php
You hike along the edge of the ocean to a beautiful beach and from there you can head up to some spectacular falls.
Also hiking around Waimea Canyon is pretty awesome.
For the Luau, we went to this one and had a good time.
http://luaukalamaku.com/
The all you can drink blue hawaii's and beer didn't hurt.
For eating in Poipu:
Keoki's Paradisr: Cheesy little restaurant with a bar overlooking a pond...but our favourite restaurant! They had the best fish taco's and nachos. Weird, I know. Also they have an awesome mud pie.
http://www.keokisparadise.com/
Other notable restaurants we ate at were the Beach House
http://www.the-beach-house.com/ and Casa D'Amici (great island take on italian)
http://www.casadiamicipoipu.com/
For other things to do, we spent some time beach hopping up near Hanalei which is nice but rainier than Poipu. We went to some coffee plantation which was pretty boring. We went to the Kilauea Lighthouse and saw whales and crap.
http://www.kilaueapoint.org/
Lucky dude. I wish I was there right now!