Select a theme. Maybe this year it can be something like along the lines of Bid Day theme and you have t-shirts with the saying on it and "Big/Lil Reveal 06." Everyone loves a t-shirt!
Everything at your event can be based on the theme. For example if you do "J'adore XYZ" everything can be Chanel-themed, and you can maybe have a tea party. For Big/Lil reveal, maybe the big/lil have matching purses and inside the purse is the name of the Big/Lil.
Or you can do an "Around the World" luncheon with each "family" responsible for a type of cuisine for a luncheon--- Italian, Chinese, Mexican, Greek, etc. Your T-shirts can feature a globe and maybe the saying "XYZ: The best sisterhood in the world."
Or go with what works-- bagels in the park and some field games. Maybe Big/Lil reveal is determined by matching t-shirts. You can play Capture the Flag or other types of outdoor games. Or make it a beach theme or pool party.
Hope this helps. Sometimes the best events are the ones that are the most casual. Maybe you can plan a sisterhood sleepover with silly games, movies, magazines and crafts where the whole chapter bonds with their new bigs/lils.