How to prove you're vaccinated as more venues make it mandatory

With new vaccine mandates in place and more on the way, individuals who wish to patronize some gyms, bars, restaurants and entertainment venues will need to prove they're inoculated against the coronavirus.

Simply saying, "I'm vaccinated," is no longer sufficient. Neither is sporting the "I got the shot" sticker you might have been given when you received one or both jabs.  So, what kind of proof is acceptable, and how can you demonstrate your immunization status without compromising the security of your personal information?