What do I have in mine you ask?
In my steel-string case: nothing!
In my classical guitar case: 2 packs of classical guitar strings, tuning fork, and a guitar de-humidifier.
In my violin case: extra strings, a tiny screwdriver, the tag that originally came with it, 2 blocks of rosin (one good, one bad) and one of those little bags of silica particles to reduce moisture--it's been in there since I got the violin in '94.

--Mbo