The core of our alumnae association is fairly small (there are 22 dues-paying members and only 10-14 or so come to events) but ages range from 22-82+. We only get together 4-5 times a year but try to do a variety of things that everyone can enjoy. Past events have included: a wine tasting, Founder's Day dinner, Christmas Tea/brunch, breadmaking, paint-your-own pottery, a Monmouth duo dinner with the Pi Phis and a garden party. But what everyone seems to like the most is just getting together, having a glass of wine or two and socializing!
There are about 4-5 of us who are all under 30 and married (or soon-to-be) who try and get together a few times a year for dinner, drinks, a movie or just hanging out. We call it "Night Owls" and I know other alumnae associations in other cities have similar groups as a good friend of mine is active in the group in Minneapolis.
I think once you get the word out and try a few activities to see what works best with the group of younger women you have, more people will be interested and come to events. The hardest thing is just taking that first step and really, all it comes down to is someone taking the initiative to plan something.
Good luck!