Kapa'a is a great place to stay. It is relatively central. So, if you have plans on the north shore, south shore, or all the way west. You are no more than an hour away from pretty much anywhere on the island, pending traffic of course.
There is a Safeway across the street for all the food and beverages you need. There are plenty of restaurants within walking distance. I HIGHLY recommend Lemongrass. Historic Kapa'a town is about 5-minute drive north and there are plenty of options there as well.
You can easily walk to the beach and enjoy the water right by the complex. Lydgate Beach is about a 5 minute car drive, Kalapaki Beach is 20, Poipu and Shipwreck are about 40 minutes away.
Great little gem of a place to stay!