Hi Liz
I did this trip in May this year and had a fantastic time. It was very hot in both Las Vegas and Palm Springs, so I suggest that you make sure you take high factor sun cream and sun hat. As it can be cooler on the west coast, I would pack a sweater or thin fleece (although we arrived in San Fransisco in a heatwave!)The temperature in Hawaii stays constant, so shorts, a skirt or cropped trousers will be fine for wearing during the day and evening.
As United Airlines are very hot on the weight limits, I would pack as little as possible and lightweight items that can be layered.
I hope that this has been of some use and I hope that you have a truly wonderful time. It really is the holiday of a lifetime!
Jane