There is so much to do on the Island of Hawaii, it's hard to know where to begin.
RELAX! You won't get it all done in a week or 10 days and that's okay. Here's a few ideas for you though.
If you want to see the majestic waterfalls of the Island, plan a tour of the north side of the island. You could use your rental car or rent a Slingshot through Aloha Motorsports and select the "Big Island" location that will give you an open-air sports car experience complete with navigation and 200hp of fun.
We also rent motorcycles if you have a motorcycle endorsement on your license.
Start off in Waikoloa at the King Shops and roll north on Highway 19 up to Highway toward 270. Along the way you can Snorkel at Beach #49 or Beach #69, or swim at the world renowned Hapuna Beach State Park. Check out Hawi and the original King Kamehameha Statue and the Pololu Valley Lookout. Cut down the 250 to Waimea and head towards Honokaa on the 19. Make sure you stop at Tex Drive-In for some of the best Malasadas on the whole island. From there you can hit Umauma Falls, Akaka Falls State Park, and stop for lunch in Hilo at Saucy Dog's or the Hilo Tavern or any number of excellent local grinds. There's more to see and plan on the map pictured below.