I bought some 15 dollar, stupid, designy, keychain pocket pen that attaches/detaches with an embedded magnet in it. Well the magnet wasn’t strong enough and kept falling off. Now it’s gone. I need a pen on me at all times, and having a full size one in pocket can get annoying, especially when bike riding. So I cut down a bic pen. And it’s great. And didn’t cost anything. Inspiration:

http://www.instructables.com/id/Make_a_Mini_Bic_Pen/