Do you have anything prepared for this? Do you say, no but come back in a week?
Do you say, yes, here are four pre-gen character sheets, let’s play a random short one-shot?
Do you say, no we’d need to spend time prepping for D&D but here’s a simpler tabletop game which uses minis and dice, let’s play this right now instead? (for example, Shadowdark)
Or something else?
I’ve been doing this for a while, so I’ve got old modules saved in my head. I also know generally what is required to run a successful one. What I would personally do is say “Yeah, sure. But I’ll need a bit to get ready. Give me 30 minutes.” I would then use a tool like 5etools to whip up a bunch of pre-generated characters and hand them out. If I had my printer handy, I’d print them. Then I’d run them through something like the Delian Tomb that Matthew Colville made (https://youtu.be/zTD2RZz6mlo?list=PLlUk42GiU2guNzWBzxn7hs8MaV7ELLCP_), a simple orc or bandit raid on a town, or whatever else I thought was cool in the moment.
I hope that helps. I’ve been DMing for over ten years now and playing for almost twenty, so Idk how well my technique would work for you.