Taverns are local events. They are a chance to be people of the past without the time and travel that Big events require. They are a great time to show off your spiffy new garb or try out some of the ones available in gold key. And, they are an opportunity to tell us more about your persona, but it's OK if you don't quite have that worked out yet. Show up and just have fun.
There are many themes a Tavern could take. We could be a group of travelers meeting around a fire in someone's back yard for the evening. We could be invited to share tea and conversation a a great house for an afternoon. We could be friends enjoying a feast at a local castle. We could get together to play board games or sing songs.
Taverns don't necessarily involve alcohol and children are always welcome. Sometimes they are a great chance to try out medieval recipes and other times participants bring whatever they would normally bring to a pot luck.
In short, Taverns are whatever the people attending them want them to be. If there is something you think would be a good theme, spread the word and make that Tavern happen.