I've recently moved to a ward that has an activity every month because we all live far apart. If you only have a week or two, keep it simple. Save the fancy stuff for when you have more time. Set up a game night in the building. Get sign-ups for food. Arrange for volleyball or basketball in the gym (our ward did that and everyone loved it, even if they didn't play), board games in a smaller room, and toys or a video in the nursery. Rotate nursery workers so everyone gets time to play.
Our ward recently had a talent show that was a big hit. It included a band made up of adults who hadn't played since high school and had to borrow instruments, with a few students who really could play. They had only a few practices, and it went over so well I think they will become a permanant ward group.
This month we're having a mystery dinner. Another month, they had a Night in Bethlehem. There are a lot of internet sites with ward activities on them. http://www.lightplanet.com/mormons/actcomm.htm http://www.primaryetc.com/activity.htm