I wouldn't worry too much about what the weather says. I'm in Waikiki now. Right before we left the forecast said it was going to rain (versus showers) all this week.

The sun has been shining since we arrived and the first hint of rain in the forecast is for Thursday. I wouldn't worry about it too much. It's likely the forecast will change.

If it does rain, often you can hop in the car, drive 15 mintues, and the weather will be different.

If you don't want to do that, I love the Bishop Museum, and Ala Moana is an excellent place to people watch & spend some money. The food court, believe it or not, isn't half bad. It's far superior to those on the mainland.