Ask people to get up, pair up, and chat for 2 minutes with each other about whatever interests them. You’ll be the timer. When 2 minutes are up, you’ll blow your whistle or make some other sound loud enough for everyone to hear.
When they hear your signal, everyone is to find a new partner and chat for the next 2 minutes.
If you have flexibility, allow enough time for everyone to have 2 minutes with every other person.
If using this at the beginning of a course or meeting, combine it with introductions. After the mixer, ask each person to give his or her name, and share something interesting they learned from someone else during the mixer.