Hi Erica,
I just thought of a few things your chapter can do for the senior who is leaving. Have all the sisters contribute 1-2 items for a "relaxation basket" (nice bath soaps/salts, aromatherapy items; with her "favorite items" or items for her new home??
Depending on the circumstance and how much time the sisters or your seniors's "Delta-pi-family" has, perhaps create a home-made basket - with her favorite colors, and scapbook type items, pre-paid phone card(s) so she can keep in touch with you.
If your really in a pinch timewise, make a simple 10 x 12 inch "scrapbook-poster" collage with your chapter's family photos, everyone's signature, little mementos (sp?) a suprise dinner/going away party, etc. (sorry this is worded funny - lol).