Wolf and I just spent a week on the Big Island staying on the Hilo side, though we flew into Kona and made it back over there one day. SusanCinSF said it before, but if you're staying on the Big Island (esp. on the Hilo side), you'll be eating a lot of meals in, which isn't a bad thing for a chowhound who cooks.
One find not usually discussed on the boards:
Sombat's Fresh Thai Cuisine
88 Kanoelehue Ave. (Waiakea Kai Plaza)
We had lunch here -- our usual of Thai food staples -- spring rolls, green curry w/ chicken, pad thai w/ chicken. Spring rolls were hot out of the frier and not greasy in the least. The green curry had a lovely thick coconut sauce (no reduced fat coconut milk here, I tell ya), with zucchini and a lot of eggplant (I'm not a fan of the eggplant). The pad thai had more sauce than I was used to, but it made it extra tasty the next day for breakfast. Service was super quick. Bonus points for giving us extra rice with our to-go box.
On the malasada debate (plain/traditional):
For me it was a tie between Tex's and Punalu'u Bakery. We happened to get Tex's hot on a Saturday morning as we headed back to the Kona airport, so they were very good -- and cheap -- 95 cents, I think. A little greasy, but not bad. Punalu'u's were made w/ their Hawaiian sweet bread dough and were quite good - $1.25 if I recall. We also had Punalu'u's lilikoi glazed and Bavarian cream filled. The lilikoi was so sweet it made my teeth hurt and the cream filling while good weirded me out slightly for not being refrigerated. It didn't stop me from eating it, but either way from here on out, I'll stick w/ the plain.
I also had the ones sold by the vendor at the far end of the Hilo Farmers market who also sells the various musubis...those malasadas (75 cents) were greasy and tough. Don't bother.
On ice cream:
Tropical Dreams versus Hilo Homemade. Hilo Homemade all the way!! We had a scoop from Gammy's in downtown Hilo - a shop bringing new meaning to "hole in the wall." Tropical Dreams Kohala Road (rocky road with a Hawaiian name) and Tahitian Vanilla just couldn't hold a waffle cone to Hilo Homemade's Toasted Coconut and Banana Storm (banana ice cream with strawberry and chocolate cookie crumbs).
On the usual places I picked up on from the boards:
Hilo Bay Cafe
315 Maka'ala Street #109
The location between an Office Max and the Wal-Mart makes it seem a bit odd, but Hilo Bay Cafe seems to be working to honor locally grown/caught ingredients and while they do feature some beers from out of state, they also have Mehana and Kona Brewing Company. My fave was the Mehana Volcano Red. The chicken pot pie (veggie pot pie, add chicken) had a nice flaky crust. Wolf had the ono fish and chips (fresh catch that day) which he said were good, but not as good as the usual cod/halibut fish and chips of his youth in Washington state. Service was good - we actually ate here twice and our server Noe remembered us the second time around.
S. Hata Bldg, downtown Hilo
Another lunch out for us. The pizza (I had the margherita) was ok. Chewy, blandish crust. It was my 3rd favorite of all the pizza we consumed. You can see my other post about Big Island pizza here: http://www.chowhound.com/topics/408215
SusanCinSF was right - you'll be eating a lot of meals in if you aren't staying at one of the resorts on the Kona side.
Foodland in Waimea was actually the nicest grocery. And they get bonus points for Big Island-raised beef. There's something not quite right about eating Safeway's Ranchers Reserve meat when you know it was flown in from the mainland and probably the same as the Safeway down the street from your house.
In Hilo proper, KTA Superstore (Puainako) gives you a more local feel.
Kuhio Grille (Prince Kuhio Plaza)
Kiawe Kitchen (Volcano)
Big Island Candies (lots of free samples - some interesting ones including chocolate covered corn chips and a chocolate crunch including potato chips).
More photos and whatnot at: http://www.dailywaffle.com