I maintain a website for an alumnae chapter (as well as the active collegiac chapter). I have been told by countless alum that the website, more then any activity, newletter, or event is what keeps them updated and interested. We post wedding pictures, baby pictures, pet pictures, etc. Those alumnae who needed a break from sorority for a few years love that they can regroup with us at any time and find out what everyone is up to.
To show appreciation for your alumnae group supporting the chapter, I would suggest an alumnae appreciation lunch to both honor all of them and as a "reunion" day for them. To reach more (those not geographically close), maybe send out cards of thanks.
I have heard of a chapter where the members of the college chapter each "adopted" an alumnae and send her cards, emails, gifts, etc. (like a secret sister). This made a connection between the alumnae who graduated long ago and those sisters still active in college.
I think it is great you want to recognize your alumnae.
Just let them know you appreciate them and they will appreciate the thought!