Cycling isn't a major activity in Hilo, and it's primarily for recreation, not for transportation. But Hilo is an eminently cyclable town. Mid-Pacific Wheels is your best bet for rentals.
The Hele-On Bus covers much of Hilo, although service is rather infrequent. There are three intra-Hilo routes that cover most of the town: the Keaukaha route includes stops at Hilo Airport, Prince Kuhio Plaza and the beaches.
Free parking is generally available around town. Downtown street parking is free for two hours (or more, since enforcement is slack); finding a spot is easy except when the Wednesday and Saturday farmers markets are on.