Just north of Hilo at mile marker 7, a small 4 mile scenic drive leads past the Hawaii Tropical Botanical Gardens.  At the Botanical Gardens,  you can enjoy not only the lush tropical rainforest and unique Hawaiian plant life, but you can also take in Onomea Falls located inside the Gardens.  A bit further north, watch for the signs for Akana Falls. Just up the road is Kolekole Park with a monument to the Tsunami of 1946 and a picnic area if you need a rest.
Just past mile marker 16 is a series of falls for the next few miles. One of these is the impressive Umauma falls which you can see from the World Botanical Gardens as you take in the splendors of Hawaii flora and wildlife. Plan to spend some time enjoying the dense rainforest and the many waterfalls of the area. You will not be disappointed.

At mile marker 42, bear off to the right onto Hwy 240 and take this road to the end to see the breath-takingly beautiful Waipio Valley from the overlook area at the end of the road. Featured in the movie, Waterworld, the black sand beach of this spectacular valley gives way to some of the most lush and gorgeous scenery in all of Hawaii. Unless you have a 4-wheel drive vehicle, all you will be able to do is peer into the valley from the overlook as the road is a very steep 25% grade into the canyon below. Take heart though because there are tours of all types from jeep to ATV to horseback tours offered into the valley below. Residents here like their privacy so it is best to go with a tour group if at all possible. You can walk to the beach at the bottom easily enough but the trek back up is definitely not for the breathing-challenged.  On a clear day, you can even see Maui off in the distance. This area is definitely worth a visit no matter what time of year you go to the Big Island.