I would talk to your doctor, but I think light stuff should be fine. Listen to your body - if you get lightheaded, couldn't carry on a conversation, or it just doesn't "feel right," then stop.
Also, don't do anything that inverts you (though I can't imagine the Wii Fit would have you doing headstands?).
Those are the criteria even for those of us who work out regularly when not pregnant, so I would think they'd be the same for you?
B, happily married to M. Mother to two boys: B (3/08) and A (9/10)