It's common knowledge that spending a few hours surrounded by trees and plants can increase your energy. If walking around town has tired you out, Gilbert Carvalho Park is a perfect place to stop and recharge your batteries.
You'll have no end of sightseeing choices in Hilo. Gilbert Carvalho Park is a great place to start here, but there are loads of other interesting places to discover. University of Hawaii at Hilo, Panaewa Rainforest Zoo, and Pacific Tsunami Museum are near the recreation and green space and will fill in your day nicely.