Movie extras

You could make them chapters at the end of the movie but that will be a fair amount of work.

You could just rip the extras you think you would want to watch more than once and make them their own self-contained video files. For example, store your movies in "movies" and store your extras as "TV Shows" making each extra you want to store an "episode".

You could rip 'em and then splice them all together and render one video of extras with each set off as a chapter. Then, you could have 2 lines for each movie in "movies": <Movie title>, <Movie title> Extras

Apple has an iTunes Extras functionality available http://www.apple.com/itunes/lp-and-extras/ but there hasn't been much in good "for dummies" authoring tools, nor have the Studios shown much interest in it for rentals/purchases (except for a small number of titles).

If it is an extra feature that I think is really good. I would do as they suggested above. I list them under the documentary category in the movie list. I usually use the original poster art for the main feature movie and the DVD or Bluray cover for the extra features that I decide to keep.