Five minutes is completely taking the p*ss. If they're working on the computers it will take some of them that long to log on. How about a 'spot the deliberate mistakes' starter using a webpage that you've set up with loads of errors on it? You could include meta tagging errors or maybe embedded items that don't display properly. The theory behind it would be you're doing an informal survey of their current state of knowledge. Alternatively, a badly-edited piece of video or animation, say 90 seconds long. The group watch it and critique it. It's about time our unions got involved in negotiating a minimum sensible length for so-called micro teach sessions.