The quandry is that you are in an area of the city that is targeted to tourists.
If you want to walk 5-10 minutes to the theater, Silvertone on Bromfield Street is a good option for American "comfort food". It's reasonable, and the staff is very friendly. It's very casual, so if you are dressed up, you might be a little overdressed. They don't accept reservations, so give yourself an extra 15-30 minutes.
Chinatown is close by, there are a ton of reasonably priced places there.
Felt (upscale billiards, and lounge) is right next to the theater, but I'm not sure how the food is.
There are a few interesting options a few blocks away on Kingston Street, literally a five minute walk from the Opera House -
Kingston Station - new, French bistro style, good food, good bar, not too expensive
Good Life - interesting food basically American with a twist and good drinks, too
JJ Foleys - burgers and beer
Give yourself a few extra minutes, walk into each one and check out the vibe and see what suits you. Prior to the show they may all be quiet, afterwards there may be a bit more lively scene if that is what your are looking for.
There is a bar and restaurant in the Hyatt, too, though I haven't tried them.
Hope this helps. I'm going to Teatro first, they're used to the pretheater crowd and get you in and out with a tasty meal.
I checked Open Table and they don't have an opening at Teatro, but sometimes they can slide you in.
If you want fancy (and a reportedly great wine list according to repected Chowers) Troquet has an opening at 5:00
Have fun, report back