E.P.
Love this kind of stuff. A couple thoughts:
1) Collect old pictures and have someone help you to put it on a slide show. The hall should have access to putting up a screen and it would be nice to see all the pictures, over the years of your mother with all of her loved ones - family, friends, etc... Or , set a lap-top or tv in a safe spot to "scroll" a bunch of pictures of her, to music.
2) For a 50th birthday party, for a centerpiece, we went to an old fashioned candy store (or you can go on-line and buy old fashioned candy from their era (Ferrara Pan Candy Company, makers of lemon heads and boston baked beans, is right in Forest Park). This was a great conversation starter. We just had cool glass container (purchased from Pier 1, who has a big sale right now, and placed them in the centers of all tables. Attach helium balloons for lots of color. Give away the centerpiece at the end of the night. Use some kind of fair way to "give it away"... maybe it will go to the person who has a birthday closest to your mom?
3) Since you have a DJ, just down load some of her favorite songs, in advance. Some special songs that you know of are fun.
4) Cute little refrigerator magnets with two pictures on them (one current/one past) is fun, with a cute saying. These can be used for party favors. Don't make them all the same. Purchase business card magnets - one side already has adhesive. You can scan and print pictures from home or color copy them at an office supply store. 100-pack at Staples is a good price.
http://www.staples.com/office/supplies/p4_Business-Card-M...
5) The DJ will have a microphone available (ask him if others can use it - sometimes they don't like that or they will bring their own stand). You may want to allow some time for others to give a tribute, tell a funny story or just say some kind words. If you want this to be memorable, let a few people know you are doing this so that they can be prepared for a little semi-impromptu moment. Even someone writing a quirky poem about her can be fun, if done in the right taste.
6) Regarding decorations: What does your mom like? Favorite colors? Favorite holidays or hobbies? Does she have a collection of anything? Famous entertainers that she likes? Put up things that remind you of her. Let your decorations tell a story. If you only have a small area, make it an "All about Mom" corner. Some guests will know her a lot better than others - it's a great time to have people celebrate and learn about her life.
7) Get cheap white or solid colored tablecloths (preferably paper top with a plastic back). Then buy a roll of banquet table cloth (40" x 100' for about $10-$15) , at any party store, in a different color and add a 40" x 40" square of color on top of it.
Good luck.