Good suggestions so far. Here is an alternate thought:

Move the food idea to the background. Hawaii is not about food. (Granted, there is some good food to be had, but mostly it is ordinary.)

Instead: go for drinks on the beach at Waikiki. You can start at the Moana Banyan courtyard and walk West to Duke's, etc. Most of these have tables essentially on and reachable from the beach. The prices aren't too bad - $4-5.50 for a beer or Mai Tai. As a bonus, many also have live Hawaiian music in the evening. So, you can listen, watch the sunset (or the surfers and beachgoers earlier) and have a few. You can also get appetizers for food if you are really hungry.