Hilo area is good to use as a base for visiting Volcanoes NP, but it sounds like you're already staying somewhere near VNP for a night. VNP is a must-see, esp. now that it's apparently erupting again.
But there are enough sights in the Hilo area to fill up at least a day. Rainbow Falls, Boiling Pots, and the area around Wailuku River, Akaka Falls, and the the Hawaii Tropical Botanical Garden are all worthwhile IMO. I'm not a big botanical garden fan in general, but that one was definitely worth it. The farmers market in Hilo is supposed to be pretty cool, but I don't think it was going on the days we were there. I think 2 nights and a day in Hilo is about right.
A couple other must-sees for me on the Big Island would be Mauna Kea (make sure you acclimate for 1+ hours while at the Visitors Center before driving to the summit) and Waipi'o Valley. We did the Waipio Valley Shuttle tour with the company that takes you down into the valley, and we found that to be very worthwhile.