We have little sis/big sis week at our chapter. The big sis leaves little gifts and notes each day on the little's door with "clues" about who the big is. One of my Beta Buddies was my little sis, and I love making elaborate signs and things, so my friends made me leave her lame tiny little signs so she wouldn't think it was me... it was great, she was completely fooled and on big sis night I gave her a huge beautiful sign to make up for all the crummy ones

We then have our revealing on Big sis night. The VP Social usually tells the pledges that they are having dinner with AAC, and then when they show up there are all of the actives, and the seniors come in wearing crazy function wear, and they make up silly poems about each big and little pair. We then usually go out to dinner with our entire Pi Phi family, and we give our littles presents -- usually a picture frame, flowers, a family tree, and a "family" gift -- sometimes something silly (my family had these horrible silk flowers that were passed on from year to year) and something serious (like a pin box).
Bigs and littles were a huge deal when I pledged in '93 because our pledgeship was 12 weeks long and you spent a ton of time with your big. I'm still great friends with mine!