Keep it simple and professional. Don't use gimmicks or games or anything like that.
Appoint someone, such as the project manager (or, in Scrum, the Scrum Master) to solicit feedback from everyone, based on team preferences. This could be via email or written on paper and turned in. Ask for everyone to contribute things they think went well and what can be done to continue, things that didn't go well and how to fix the problems, and general thoughts or comments about the Sprint.
After all the ideas are collected, put them on a tri-fold poster, or make a slide deck, or just handouts. The entire team goes to a conference room, sits around a table, and goes through every point, coming to agreement about how to move forward, fixing problems and keeping the good things going.
Don't forget to timebox the meeting. In a 1 month (4 week) Sprint, Scrum has a timebox of 3 hours. Pick an appropriate length of time for your meeting, and stay focused for that time.