Castmembers can bring 1 guest to the Appreciation Day events (I'm not sure if more are allowed at any time, but seemed all were +1). I went to one with my Aunt for her 20th year at Prime Time Cafe (it was at MK). Lots of free food, drinks, short lines on rides, special fireworks show, and lots of fun interaction with costumed CMs that's slightly different from normal park interaction (they can be a little more fun, or sarcastic, because everyone there is an 'insider'). There are usually events for every 5 year milestone - 10, 15, 20, 25, etc years. She had been to a few and would bring her local family living up there - that particular year I happened to be visiting Disney during that event and she invited me along.
I also got to do a few cast member early previews of some of the rides before they let the public do any previews, and of course the wonderful cast member Property Control sales - where you can buy Disney merchandise for ridiculous prices (up to 90% off) - overstock, discontinued items, etc are super cheap.