My recently departed bass case had a "thank you" sign taped to the inside of the lid, for acknowledging tips, and spare strings, big old thumbpick for when I didn't play with my fingers, various allen keys and other guitar tools, strap, spare nine volt battery, eq peddle, cheat sheets...

My mandolin case is lined with an old union t-shirt.

Other cases have the usual strap, picks, strings, tools, music, post-it notes, rags, capos and the like, nothing particularly fanciful.

Years ago I worked with a banjo/etc player who always carried condoms, smokes, rye, and snacks among other things more lifestyle than music oriented in his cases.

Peace, Mooh.