The Big Island of Hawaii has two sides. Most of us tourists stay on the Kona side. There are far more places to stay there. There are more restaurants. There’s more people. Don’t feel bad about Hilo, it gets more rain! Well, that means more waterfalls! Several parks are either in or within half hour of Hilo. These are reasonable things you can do with a day in Hilo.
A half hour North of Hilo is a very special place. As you need a four wheel drive truck to go in special. You rental car will take you to a great vantage point overlooking Waipio Valley. A tube shaped valley framed bit steep walls. Inside the roads are streams. Ribbon waterfalls are everywhere. A black beach is down there too. Important part here. The locals hate you. If you get the chance to go down the hill, make a right at the bottom of the road between a pair of guard rails. There may not be any good signage. Currently the area is closed due to dengue fever. The picture used is from Wikipedia.