Indoor Games
Classic wet weather option. A good topic to hold because it can keep most people occupied and enjoying themselves at once.
Materials Required
As many board games and rules as you can get your hands on. Basically if you have the rules and equipment for a medieval board game bring them. There are also enough dice games around to justify bringing a few along. Cards can also be useful, and quite justifiable late in period as most cards use the wife of Henry VII as their depiction for the queen.
Meeting Outline
There are a couple of ways to run this, but my usual method is to start off with everyone involved in a round of "Shut the Box" as described in the Known World Handbook. I then move on to describe Nine Mens Morris and a few of the variations of chess, encouraging people to play once they get interested.
The other alternative is to just set up the games around the rooms with instructions next to them allowing people to play what interests them.
Possible Problems
Getting your hands on boards can be a problem. Ask around beforehand though, and you should be able to get enough chess boards to the meeting. If you really want to play Nine Men's Morris but don't have a board available, just draw out the board on a piece of paper and use the counters from a modern day board game.
Finding partners for everybody to play with might be difficult. Once everyone else is occupied though, feel free to play with somebody who can't find a partner, but make it clear to whoever you are partner to that you might be called away a couple of times during the course of the game to interpret rules. Bear in mind that some people prefer to watch people playing rather than join in. If somebody says they're happy to watch don't force them to play, they'll be happier that way.
Sometimes the rules of a game are ambiguous. There is no real way around this except to play test things. If you're still not sure don't worry, it was almost certainly played more than one way in period. This isn't license to make wholesale changes to a game, but a good excuse for minor inconsistancies between methods of playing.
Other Options
Ever considered the joys of live chess?
If you like, you could go into the history of games a little.
Resources and References
The known world handbook contains enough games to take up an entire meeting easily. In addition there are a number of SCA and SCA related publications that may be helpful, such as Compleat Anachronist #4, "How to While Away a Siege".
There are also some resources of use on-line:
Webbed by Conrad Leviston. Please mail any suggestions/corrections/comments