This music video of a girl playing Garbage's "Only Happy When it Rains" (embedded in the continuation of this post) is actually an ad for an iPad app called OMGuitar, but that doesn't make it any less impressive -- the app looks pretty good. It allows you to hit a button to choose whatever chord you want to play, and then you strum along on virtual strings to your heart's content. The app also offers up some simple guitar effects to play around with as well.
Unfortunately, since you're playing with an iPad, there's no tactile feedback, so I'd imagine you'd have to practice a bit to know exactly where to put your fingers and when. But given a few chords and a little bit of time with the app, it seems like it would be pretty easy to strum out a few of your favorite popular songs this weekend.