i've made bongs/pipes out of:

hollahoops
ceramic figures (both large and small)
goldfish bowls
socket wrenchs
metal plumbing supplies
beer bottles
aluminum cans
drink bottles
plastic containers (other then used for storing drinks ex: nyquil bottles, pill bottles ect)
blow-dart guns
(i have personally seen guns used, but have not made one out of a gun myself)

there's a few more things you can add