Not knowing your tastes, I'm going to offer some wildly different (but not expensive) suggestions:
** Montien at 63 Stuart Street -- excellent Thai food (and sushi)
** Legal Seafood - I prefer its Long Wharf restaurant over the Park Square locale, but Park Square is closer to your theatre
But you need not confine yourself to the Charles Playhouse' immediate neighborhood. Boston is compact -- very walkable and well served by its underground train system (the "T"). So if there's a particular kind of cuisine or atmosphere you wish -- or if you're not adverse to spending more money -- post back so folks can offer more specific recommendations.
Have a wonderful visit!
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