This is not really a town in any sense but a collection of large hotels and a shopping mall with restaurants. The only reason for going out here is to do some of the activities offered through the hotel or visiting black rock for snorkeling.
Kannapali is a resort development. One main little mall with shops and restaurants for you to check out during the day. There is also the Whalers Village Museum where you can learn all about the whaling industry in Hawaii.
Then in the evening there are things happening at the resorts. These change often so listing them is counter-productive. Again your best bet to find out what is happening during the nighttime for entertainment is to check with the hotel concierge. You don’t need to actually be staying in the hotel to find out from them what’s going on that week.
The best things to do during the day is to spend it out on the water, in the water or beside the water, lazing on a lounge chair or at the pool side cabana.
At the end of the beach in Kannapali is Black Rock which is a great place to enjoy snorkeling. There are normally tons of turtles and wildlife all along the rocks at the end of the beach. The Sheraton Hotel definitely has a prime spot. Their pool is pretty great too.
There are luaus up and down the beach each evening. If you want to, you can easily hit a different one each night, with different food and entertainment.
Here are some interesting links for you! Enjoy your stay :)