We would split into several small groups. Each group would be assigned a different web micro-framework to use to build a blog (or something else, but everyone should be building the same thing). After X amount of time (1 hour?), each group will discuss with the other groups what they liked or did not like about their micro-framework.
Presentations on micro-frameworks are well and good, but when does anyone have the time to really use it? When was the last time someone pointed out a micro-framework (not named Sinatra), you thought it looked cool, and you actually built something with it. Probably never. This way we'll all learn something about a micro-framework because we'll have actually worked with one. Perhaps we'll even be inspired to use it again, or use one of the other group's assigned micro-frameworks.
Not Sinatra. Sinatra is something everyone has used, at this point. However, if you have any other !Sinatra recommendations please make them. (Padrino is Sinatra, and it's also not a micro-framework.) (... I could be talked into Sinatra for a group comprised of people who have never used Sinatra, especially if that means more, smaller groups.)