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.
Green Sand Beach is close to South Point, so if you are going there, don't miss the green sand beach. You have to park and hike to it, so take plenty of water, food, sunscreen, towels, good walking shoes and hats. Seeing it once was enough for me, but it was worth it....