Aloha again, Goofynut!
Okay, I can't swear to what the heck the weather will be on our 5 sea days returning to the mainland. I did the 14 nt R/T from LA to Hawaii in 2012 and it was actually quite chilly until we got within a day, day and a half of Hawaii. I would assume that we will get less than warm weather as we get farther away from Hawaii and farther north. Definitely bring stuff to layer.
Hawaii? Wow. This loveliest of U.S. states has every microclimate known to this planet save two.
At sea level the temperatures are pretty static year round on the leeward sides of the islands (the much drier western sides of the islands), averaging around 81 degrees F. Gorgeous temps! Winter brings more rain and the windward sides of the islands are rainier by far, which is why they are so verdant and gorgeous!
And remember that the highest mountains on the earth are the mountains of Hawaii. Yep, that's right. They actually start from the sea floor, but even above sea level, Haleakala on Maui is over 10,000 ft above sea level and Mauna Kea and Mauna Loa on the Big Island are about 13,000 ft elevation! WOW! So in just an hour or 2 one can drive from beautiful beach weather to where you need a serious coat and mittens! For real! Snow is common on the peaks of the BI mountains in winter. Haleakala gets occasional snow as well.
Expect gorgeous weather for most of your sightseeing. Bringing a light, water resistant jacket is a very good idea. And as said, clothing that can layer for warmth as needed. When we drive upcountry, I always take a jacket. We love the amazing variety that Hawaii offers. IMHO it is truly the most beautiful place God made on this planet. I cannot WAIT to return!!