We were on the Big Island in December one year. Didn't need a jacket in Hilo, but by the time we got to Volcano, we did. Hilo was nice--we stayed at a small hotel there called The Dolphin Bay Hotel. This is anything but a resort; it is a mom-and-pop small hotel with a very warm Hawaiian welcome. Homemade muffins and coffee in the morning, and fruit picked off of their own trees in back. You can hear a stream babbling at night through your open window. We used it as a base to tour the whole island. Hilo is not a resort town but more of a normal working town with some nice little restaurants and the U. of Hawaii. The eastern shore is not beach but more rocky with giant waves hitting them in a spectacular display.
The Big Island isn't so big. It take two hours or so to go from one side to the other. The east shore is lush and tropical and the west is beaches and sunsets. Highlights were Volcano National Park and going back at night to an area southeast of there to observe lava running into the sea. Kilauea Iki--a dry lava bed you can walk across. Akaka Falls north of Hilo. Sugar plantations and ranch land to the north. The west has some interesting historical spots like a city of refuge. Southwest there is a black sand beach with giant sea turtles. Also on the western shore are coffee plantations you can tour. Back in Hilo you can see Rainbow Falls and the Lyman museum--a relatively small museum full of cultural and natural artifacts, and a historic mission house.