I've been to all 4 major islands. Winston's description of them is pretty spot on. And having been to Pearl Harbor, I will probably never go back to that island for more than 1 night - when DD is old enough to understand the history behind Pearl Harbor. Oahu is very overdeveloped. Consider flying into Oahu for 1 night, go to Pearl Harbor the first morning (you can reserve your spots online - it is free for Pearl Harbor, but you must pay to visit the Missouri. It's optional, but worth it.) Then fly out to the island you want to stay at for the remainder of your trip.
My advice is to get a Hawaii tour book and read about the different islands and find the right one(s) for you. If you are only going to be there for 5 or 6 nights, I recommend sticking to 1 island or do the first night in Oahu and then the rest of your stay at another island. Any island you stay at will have a good mix of hiking and beaches to explore. You will also need a car, as their public transportation leaves a lot to be desired.
H and I are looking to go back in 2019 before DD turns 2. We will do a week on the BI and a week in Kauai. And the BI is BIG! Kailua-Kona and Hilo are the two major towns there and it is a 3 hour drive from each to get to the volcano. It also get pitch black on the drives back. There is a hotel at the National Park. Our plan for the week will be to fly in and spend a few nights in Hilo. Go to the Volcano for 1 night (to see the Caldera glow - we missed it the first time!) and then the remainder of our week will be in Kailua-Kona.
Kauai is our favorite island. I almost couldn't get H to come back from our honeymoon there! It is also the most expensive island. When we have gone, we always pack enough sunscreen for our entire trip. We also pack a bag of cereal and some rice for meals.
Once you find your island, come back and let us know! I'm sure we can help you plan out the rest of your week!