I went through informal for Phi Mu. After attending 3 nights of events (I was invited by some sisters who lived on my floor), I went to their room to thank them for inviting me to the parties (on my way to class).
Amber (sister # 1): Oh you are going to class?
Suzanne (sister # 2): This early?
Me: Yeah I wanted to get something to eat so I am leaving early
Amber (sister # 1): In that case, here (hands me my Bid Card)
Basically I had almost messed them up by leaving for class early

My screams of joy could be heard throughout the whole building. There was an RSVP request in then enevelope to call the rush chair and accept. I did and my Lion Buddy showed up later in the afternoon with goodies and took me to the Bid Day party on the hall. So I think it's a great idea to surprise them at their dorm!