I, too, like to stash away as much sweet, sweet per diem as possible, while also being compelled towards mandatory fun and microbrewerys.
For trips over 1 week, I go out to dinner the first 1-2 nights, then 1-2 times each of the following weeks, and definitely the final night. The other nights, I tell everyone I'm introverting, and need to recharge.
For 1 week trips, I go out the first 1-2 dinners, as well as the final night.
Less than 2 week, I might try to skip one dinner, but in general I simply go out every night.
As for food selection, I try to steer towards sushi, which can usually be bought either al a cart, or a single economical roll. If sushi isn't acceptable, I make sure to order something that will allow leftovers. That neatly solves the lunch issue. "Sorry, I've got leftovers. Wasting food makes me feel so guilty." I also avoid alcohol, unless it's the last night.
This approach has always allowed me to bank some per diem, but never as much as I'd like. My coworkers see me as socially capable, while also thinking my introversion runs deep. Which is the ultimate indicator of a true compromise - equal unhappiness on all sides.