We went to Hawaii two times in the past two years by plane (cannot afford the cruise ). I have found that July/Aug is way too hot (may be cooler than Las Vegas ). When we were there, the cab driver told us that the best time should be late Oct and Nov because the weather is getting cooler and hotel prices are cheaper. Christmas time is a zoo right there.
If you are going to Honolulu/Oahu, the cruise ship docks near downtown area, not much to see. Waikiki is about 30 minutes away by bus.