Many thanks for all your helpful responses - I just got back, and these suggestions were invaluable! The food on campus did mostly suck, though the vegetarian options in the sustainability courtyard were a good find - thanks. I ate in quite a few places on King St, and ended up renting a bike for some of the time, which made places like Ono's and Leonard's an easy option. In the end I did very well for food, thanks to these tips.
Another place that you should check out for a lunch/dinner pick-up would be Nijiya Supermarket (on University/Beretania). They have really great bentos that they make on site. Itʻs also a great place to pick up fun Japanese snacks and drinks.
When the weather gets too warm for you, Yogurtlandʻs (on University, right next to YMCA across the street from campus) frozen yogurt may be your salvation.
Kaimuki Man did a great job of covering the many options in and around the UH area.
A good breakfast place (and healthy lunches) he referred to, but didn't mention specifically, is Kaila's in the Market City Shopping Center, corner of King and Kapi'olani, where the H1 freeway intersects, and Kapahulu begins (or ends).
He's right about the generally poor choices on campus, especially in the summer, however, there are healthy plate lunches in the "Sustainability Courtyard" near the center of campus where two vegetarian outfits offer reasonable, tasty, and healthy options...although both may not be operating during summer session.
Another healthy lunch/dinner option around University/King is the health food store Kokua Coop, with sandwiches ready to go, and a hot buffet.
Kokua is open 8:30 till 8:30.
Kirin is one of the best Chinese restaurants in Honolulu, and that makes it pretty darn good...the crispy skin chicken is not to be missed, and their chicken/chive potstickers are wonderful.
KM didn't mention Mr. Ojisan on Kapahulu...pretty good local style Japanese food, especially good ramen.
I had hopes that Catch of the Day would actually sell fresh seafood as their sign implies, but they seem to be mainly a plate lunch place with lots of poke (marinated raw fish salad...very popular)...not sure about the quality, but the place was full the one lunchtime I looked in there recently. Good poke is also available at Tamura's, at 9th and Waialae, which is also has the best wine selection in town, and a limited but high quality selection of gourmet foods, lots of liquor, and beer.
Spices has a lunch special which they don't promote very well, but makes a delicious and reasonable sit-down lunch Tues-Fri. Their regular menu for both lunch and dinner is fine for several people, but not a bargain for one. However, the food is consistently good, with some unusual items not found at other places...the Laotian sausage is spicy, and definitely worth trying.
Macaraca has some great comments. I haven't been to Kalia's, but have heard good things about it. Ojisan was ommitted intentionally because of personal experiences there, as was Dosai, an Indian place near Ojisan. Glad to hear about catch of the day, was wondering how it was. And agree completely about the Spices special.
Between Tamura's and Fujioka's the wine selection in Kaimuki is really good, and both of them have some gourmet foods, with Tamura's being more extensive. Fujioka's, in Market City is closer to UH, which may not make much of a difference unless you are on foot. Also in Market City is Formaggio, I've not been in a long time, it gets varied reviews.
Where are you going to be staying? The food on campus generally sucks, although if Hamilton Snack bar is open there are a few more choices. Learn to love Loco Moco's (a large scoop of rice topped with a hamburger patty and a fried egg topped with brown gravy. Sure to clog your arteries.
For dinner you are in luck. There are many places in the Moiliili, Kapahulu, Kaimuki areas, all within walking or bus distance. Moiliili radiates out from the intersection of University and King. Within a two block radius there is decent greek, indian, thai/fusion, japanese, vietnamese, chinese, and american food. (Greek Corner, Maharani, Spices, various japanese, Pho South King, Kirin, and various american - buffalo boys hoagies gets my most recent vote). Just over the hill toward Kapahulu/Kaimuki are Gina's Korean BBQ, Graces Drive in, Banana Leaf Cafe, and a couple more in Market City.
Heading down to Kapahulu (walk, busses infrequent - route 14) there are Leonards Bakery (malasadas), Wassabi Bistro ($) Burgers on the Edge, Good to Grill, Caliente Del Sol, Ono's Hawaiian, Pyramids, Taste of the Bayou, and Rainbow Drive-in to name a few.
Going up Waialae into Kaimuki are TajMahal, Sabrina's, The Fat Greek, Catch of the Day (I have yet to try), House of Garlic ($), then a long break and then the major Kaimuki Shopping district with a couple of dozen more places. Town, Wild Ginger, Cafe Miro, Champa Thai, a bakery who's name I forget, Himalayan Kitchen, Happy Days Chinese, Cafe Laufer, Big City Diner, a vietnamese restaurant, Verbano Italian, Kim Chee II (not so great), Coffee Talk, Azteca (yum), Joses (bleah), 3660 ($), Hale Vietnam and 12th Avenue Grill.
That should be enough to get you started.