All of the above (so I won't repeat it). One expansion of an earlier suggestion which I've witnessed at work. After each performance everybody has to make one positive comment and one aspect for improvement. If it's set out at the start that they're the rules then everyone is ready for it.

However, the temptation is to hold back on the "aspects for improvement", hoping that the others will reciprocate when it's their turn.

It can work; especially if there are a dozen or so people and, say, nine of them offer variations of the same observation.