I already don't go out to restaurants, so I'll extend the challenge to "any food or drink not purchased from a grocery store to be consumed later".
That said, I've started bringing my own water when I go out, but I still need another bottle or two for that to have a good supply. I know a place that sells water bottles for $1, so my goal this month will be to spend no more than $2 on non-grocery food/drink.