I just put some tools (3 tire levers) and spare parts (valve caps, a schrader valve core, zip ties...) in my long 26.0 diameter bicycle seatpost. I then put a spare handlebar cap into the end to keep the stuff from falling into my frame.
It is a very simple idea, but I think it has potential, maybe hiding other useful things in the tubes of a bicycle.
Step 1: Laying Out the Stuff
Step 2: Put the Stuff in the Post
Step 3: Cap It Up
No one is the wiser but you...