1-Bring your own coin, just like the old days. Be sure it is only touched by the person who brought it.
2-Play Rock, Paper, Scissors. Rules: https://en.wikipedia.org/wiki/Rock_paper_scissors
According to Wikipedia, it may not be completely random if you play it many times against the same opponent (although it should be good enough in our situation): “Unlike truly random selection methods, however, rock paper scissors can be played with a degree of skill by recognizing and exploiting non-random behavior in opponents”
3-Bring your phone and use the website JustFlipaCoin.com
4-Download and use a coin flipper app on your phone.
One of the best ad-free iPhone ones is simply called “Coin Flip” by Louise Gebran, and is available for iPhone at https://apps.apple.com/us/app/coin-flip/id1078160355
For Samsung and other Android phones (and watches!) check out Simple Coin Flip at https://play.google.com/store/apps/details?id=com.banasiak.coinflip&hl=en_CA&gl=US