If you want a VERY easy Waterfall to see in Hilo, head to Rainbow Falls. It is in town, easy to park, and then you walk right to the falls. No hiking or even much walking required! It is beautiful too!
Now there were lots of people not paying attention to the signs and climbing out near the waterfall on the rocks. This is incredibly dangerous. They have lots of deaths because people do this. In fact, they were working on a recovery mission just upstream while we were there. Simply follow the rules and stay safe. You can see the waterfall really well, without endangering your life.
It is an easy, quick, and beautiful stop while in the Hilo area.