Big Island!!
Been to Oahu, Molokai, Maui, Kauai, and the Big Island. I've been 9 times to the Big Island. It's the variety that keeps me coming back, as recently as last March.
Burned out - been there: lunch, a bottle of wine, and a few hours on a sunny, warm beach recharges my interest in life pretty fast. A couple or three days of that, and I'm ready for some adventure.
Kona side (west) is lava desert. If you fly into Kona, you'll think you got on the wrong plane! Kona is the tourist haven, parties, dancing, food, etc. Very high end hotels just north of Kona. Hilo side (east) is wet, not so touristy, more like regular life. Fabulous farmer's market downtown on Wednesdays and Saturdays. It's got a U of Hawaii campus. Waimea (kind of north central) is exactly on the transition from wet to dry - the east side of the town is wet, the west side is dry. It has one of the island's very best restaurants, Merrimans. Excellent cuisine, and not necessarily expensive. Lunch will fit most budgets. Go there, you will not regret it.
Volcano - Go drive down Chain of Craters road and see the lava flow. Go at night and see the red glow all the way up the slope to the crater, and the bluish flicker of methane fires among the rocks. For a thrill, take a flashlight and sacrificial shoes and walk the lava fields at night. For a real thrill (maybe your last), walk out (at night, the park rangers will stop you in daytime) to where the lava flows into the sea. This is dangerous. My wife and I did this on our first visit when we were young and stupid. We wouldn't have missed it, but it could bag you your very own Darwin Award.
Drive over Saddle Road, goes across the island between the two main volcanic peaks. About half way across, turn north and go to the observatory Visitor Center at 9,000 feet. Check the schedule, they usually have a bunch of telescopes out for you to use in the evening. There are tours of the telescopes above at nearly 14,000 feet. The view is incredible.
Or turn the other direction off Saddle Road and go to the NOAA observatory. No tours that I know of, but the drive goes through the most amazing lava fields.
Visit the Waipio Valley (northeast), take a picnic and walk down the road into the valley. You can drive *if* you have a 4WD, which I recommend just on general principles. There are lots of accessible roads that a 2WD can't do.
Great seafood, incredible fresh fruit, your choice of environments from prehistoric rain forest to rock desert, activities from slug-like sun-warmed torpor to ocean kayaking to all-night partying.
That said, I have enjoyed my visits to the other islands very well. It's just that the Big Island has more to do, and more space to do it in.
I hope you can go soon, and that you enjoy it. Good luck.