Location: 15 miles north of Hilo
Akaka Falls is probably one of the most popular falls in Hawaii, so you should avoid going there on the weekends and during rush-hour, which is basically every day after 10am until 4pm I would guess.
I would also recommend a short stroll in the park (about 20 minutes) through lush vegetation and to another waterfall, certainly as high as Akaka Falls, but without a nice view on it, because you don't have access to the frontside of the falls.
Anyway...Akaka Falls is defenitely a must see with its 420 feet of free fall.
Big Island of Hawaii
Akaka Falls / Pololu Valley / Rainbow Falls / South Coast / Volcano National Park / Waipio Valley