You could do a cold reading, or a mind reading routine. They are generally super easy, powerful, and impromptu (or atleast the one I perform is). I generally perform one when someone catches me off guard, or with a very large audience and to a heckler (though I don't make it strange for the audience).
Any "cold reading" or "mind reading" routine that is "super easy" and "impromptu" probably isn't that good. Cold reading is a skill that requires a bit of learning and practice and mind reading is something more than simply guessing which card they have or what word they wrote down. When performed correctly, those routines shouldn't get a "how did he do that trick?" response, but should send shivers down the audience's spine because they are thinking "he really just did that."
... but are there any that you would suggest for a stranger audience tat they will enjoy?
In the other thread, you mentioned close-up magic for a large audience. What is the set-up? Do you have a table? Is the audience in front of you so they can see the table? Is there a video stream to monitors? Are you doing stand up magic? Is the audience seated in folding chairs, around tables or in an auditorium? You said you were performing while they set up the stage. What event will follow your performance? How big is the audience? All this affects the selection of effects to perform.
You also said you were performing in a couple of days. How long do you actually have to prepare? Is that really long enough to learn something new? For me, it takes several months to take an effect, master it, develop the presentation, reherase it, practice it in front of family and friends and get it to the level that I'm comfortable to perform it in front of a real audience.
With that in mind, what effects do you perform regularly? Those are the effects that you will perform best rather than something new that you haven't worked the kinks out of.
Finally, a word of kind advice. Never agree to do a show that you don't have. Too often I hear of people who do street magic agreeing to do a kids show or guy who do card tricks agreeing to do a stage show. I'm glad to help you with this show, because I want you to suceed for your sake and for the sake of the art of magic. BUT, in the future, develop your show before you agree to do it.