I assume you mean empty or dried out, no ink left.
You can cut match heads off book matches and stuff them real tight
into the barrels, they make interesting little bottle rockets!
You can saw them up and make beadlike necklaces from them!
Glue them to the outside of Pringles Cans, paint and make them into
Stick them into drainless planters to use to check water levels in the
The larger barrel diameters make great handles for use on things like
If you really want to get gung ho, most pen barrels are low melting
point plastic and can be used as potting material or poured into molds
to make small items like toy soldiers, etc.
If you want to recycle, you can remove the nylon or steel ball from
the brass ferule and save all those ferules (they bring about $1.10 a
lb. right now. By the time you get a pound of them, they may be
double that, hi hi).