Have a few pieces in your bag. These should be pieces that can be played well with minimal practice time and sound good.
I keep a binder of “gig” music that’s all very easy, sounds good and is easy to get under my fingers (45+ minutes of music that can be brought back in a few hours of practicing).
In addition, when asked to play “anything!” you can have a few pieces memorized to whip out whenever. It eliminates that awkward, “Well…I’m working on some things right now…”