Akaka Falls is about 16 miles north of Hilo and is definitely worth the hike. I would rate the hike moderately strenuous and there are are few stairs. This is one of the most beautiful waterfalls that you can actually get fairly close to.
Rainbow Falls is one of the easiest to get to and is a very easy walk - they call it the drive-up waterfall! It's right in Hilo off of Waianuenue Ave. If you can see it early in the morning, you have a better chance of seeing rainbows!
Hapuna Beach is one of the best white sand beaches on the island - wide and long and great swimming area. It is on the "Kona side" of the island, so about 2 hours drive from here, it's a nice day trip.
Some of the best snorkeling can be found on a sunny day here at the Waiopae tidepools of Kapoho. Off of Hiway 137 (the Red Road), take Kapoho Kai Road and park at the end. Walk through the jungle trail to the lava lined beach. You will see many different kinds of fish and colorful coral. It's like swimming...