I can't comment on Oahu, I tried my best to stay off that shitty island.
If you tl;dr the rest, buy Big Island Revealed. Locals hate this book because it had almost everything on the island.
http://www.amazon.com/Hawaii-Big-Island-Revealed-Guidebook/dp/0971727945
Rent a car during your time on the Big Island. There is a highway that goes around the island, but the volcano has made the southern tip impassable so if you want to go to the southwest corner of the island, you have to drive all the way around.
Get a good pair or two of swimming trunks that you can walk in all day. It rains on Hilo side so damn much that you might as well wear em all the time (the locals do). Get some good slippers.. flippers? Is that what we call em on the mainland?
. Regular shoes will get wet and you'll have to take em off anyway. Something with some guts. I wore Chacos and they were great.
http://www.zappos.com/chaco-z-1-unaweep-black
Plan for one day at the volcano. Make it the first day so you can go back if you want. Take a tour, it can be EXTREMELY dangerous (like death dangerous). Go at night, the difference is awesome. You can't really see the flow during the day but at night it's amazing. Wear your real shoes for this one.
Spend at least one day driving around in Pahoa/southwest. It's the jungle side. Mackenzie State Park is a MUST SEE. Eat lunch in rad Pahoa town. STAY THE
AWAY FROM ISAAC HALE PARK. Tourists are not welcome there PERIOD. Try to sneak into Champagne Ponds or hit up Kapoho pools for a swim. Relax. Enjoy the drive.
Hilo is so-so for tourists, but it has Rainbow Falls. All the tourists jump out, take a picture, and keep going. One of the best spots on the island is right above Rainbow Falls. Drive up (or walk around using the road) to just above the falls. There's probably the best swimming on the island. Great place to jump in with 10, 20, and 30 foot jump spots (depending on how brave you feel). AWESOME spot. It's one of two secret spots of mine not in the guide book. Locals and college students hang out up there. You're on the right path in when you see the gigantic banyan tree.
Richardson Park in Hilo was the closest water to me and I think I saw turtles every damn time I got in. There's also a beach on the south side that they love to sleep on, but I can't remember the name off hand.
I think Waipi'o Valley overlook was over rated.
I lived in Pahoa and Hilo and didn't spend much time in Kona (damn tourists
) so I can't comment there. I remember Captain Cook something or other being good for snorkeling. Watch out for urchin. They hurt. Srsly. Hapuna is the best beach on the big island and should be your "beach day" beach. It's on Kona side. When you get there go out on the sand and keep walking to the right. There's a condo or some hotel over there that's got the best part of the beach but anyone can go there.
If you have any specific questions about specific places, just let me know.