I can only respond about the places I've eaten, HRH wasn't open then. Also, we went during off season.
At Island of Adventure:
Mythos: upscale(not stuffy), varied menu, pricey, delicious (except fish),incredible themeing, accepts res. only open until 3pm certain times of the year.
Confiscos: sit down meal for little more than counter service, varied menu, large portions. There's a great character lunch from 12-2pm. Accepts res.except certain times. Not open for dinner.
Green eggs and ham: counter service tasty breakfast sandwiches, moderately priced. Also serves burgers and a few other things.
Thunder Falls Terrace: (cs), very good rice, salad and roasted corn; also chicken and ribs,wings. Very resonable prices, outside terrace is close to the end of the Jurassic Park ride.
Enchanted Oak Tavern: (cs), similar to Thunder Falls food but not as good. Interior is very neat though, worth a look.
Croissant Moon Bakery: tasty pastries/cake/coffee, also has sandwhiches.
Monster Cafe: (cs)mushy pasta, neat themeing if you like old monster movies.
International Food Bazaar: (cs)typical tired park food, differant choices (amer.,chinese,etc) resonable prices.
Beverly Hills Boulangerie: (cs) tasty pastries/cakes/coffee (tended to be dry after 3pm),ok breakfast sandwhiches, typical prices.
Animal Crackers: (cs), pricey, larger portions, menu mostly sausages and hot dogs, has kids meals.
The only place we ate here was at Jimmy Buffets Margaritaville. They take reservations. We went on our first afternoon, was a walk in. Went one night about 8pm, no res. 60 min wait but we showed our hotel key and only waited 15 min. We had nachos(huge! enough for 2 for dinner), veggie burger (good), burger in paradise (good not great), and margaritas (the mangoe one was AWESOME).
Hope this helps. If you want specific info about breakfast etc just post again. Maybe recent guests of the HRH can answer specific questions.
Also an excellent book with descriptions of almost everything about USO/citywalk/etc is Kelly Monaghan's "Universal Studios Escape; The Ultimate Guide to the Ultimate Theme Park"