If anyone hasn’t seen it, there is a thread with some ideas on how to do it: Recite the alphabet backwards
A bar trick that some friends did once is they went up to some random people and asked them to write down random binary digits on a napkin. Then they gave the napkin to me. (They didn’t tell me in advance what they were going to do.) I memorized the digits, and then they called the people over while I recited them back. Memorizing a few dozen binary digits is easy to do, but is unusual enough to be entertaining. It’s a funny way to meet new people. You can also recite the digits backwards for extra effect.
Another one is memorizing digits of pi with positions. Then you can have people ask you questions like “what is the 257th digit of pi?” You can answer, and they can check the answer on their phones. Few people want to listen to someone recite hundreds of digits of pi, but pulling an answer like that out of the air just takes a moment.