Don’t bother with the car upgrade on Oahu, as it won’t get you anywhere you can’t got with a cheap compact or fancy convertible. But definitely do it on Maui. If you’re only on each island for a few nights, you’ll need to make the most of your time. I suggest getting the Revealed books for each of your islands and start reading.
For Oahu, I think the highlights are a drive around the island with a stop at a shrimp truck (right near you at Turtle Bay), the Pali, blowhole, and other key sights outlined in the Revealed books that are pretty much stop-and-go quickly. If you time it so you avoid Honolulu during a rush hour, it’s amazing. Pearl Harbor is a must, but plan to spend the entire day if you want to see everything out there – Bowfin (awesomely maintained), Missouri, and the Aviation Museum.
I agree with PP about the Aloha Bowl flea market/swap meet or whatever you want to call it with the best deals for trinkets and food to take back to the mainland. You’ll save big bucks there if you are the type to buy that sort of stuff.
Also, use the time difference to your advantage! You’ll naturally get up before dawn, and we loved this. It meant that even though we were “sleeping in” for Eastern, we were ready to make the most of the days in Hawaii. Considering how early everything out there closes, that’s a good thing.
I will respectfully disagree with Manoa Falls, but that’s because I considered it to be a nightmare. It is a rainforest, so talk about miserable humidity. Not to mention it was crowded with tourists who weren’t prepared to climb over the boulders. Of course, my first warning should have been when a group of guys walked out with mud up to their knees. You’d think that for all the mud and ick, we’d get some great waterfall shots? Haha, nope. If my guy had climbed up the rock and peed, it would have been a bigger stream. I admit that my experience was nothing like the travel books I read. I can tell you that I’ve never had the desire to chop and burn down a rain forest as much in my life. The only thing that could have been worse would have been monkeys on the loose.
However, Diamond Head, go for it! Love it, even though it is a crowded hike. Start super early in the morning when they open and then go for breakfast at the Diamond Head Market & Grill. Amazing coconut pancakes. Indulgent loco moco. It’s okay since the calories won’t count after the staircase. 🙂
If you’re thinking of a Lost tour, those can be fun. But, there is no formal set. There is a website that outlines many of the different filming locations around the island. We were huge fans, so I used that to plot some interesting sites for our days driving around. Some of the silly things were like noting where the gas station was that sold Hurley his unlucky lotto ticket since we needed to fill up anyway. While we were there, we grabbed a soda from the nearby Mr. Cluck’s. (R.I.P. Tricia Tanaka) When it came to spending any money, we did the Hummer Lost/other movie tour. We opted for it because we figured we were really paying to be entertained, and the vehicle requires a small group. It was fun, our guide was funny, and we did some offroading that many of the other tours didn’t do. In fact, we timed that so that we were dropped off in Waikiki and did our stroll along the shops and beach all day afterwards. We caught the free hula performance on the beach, and finished our night with drinks at the bar that stood in for the Sydney airport bar that overlooks the marina there. It was quiet and lovely. With strong drinks, which makes it all good.