Other objects are printed in an implementation-dependent manner. It is not required that an implementation print those objects readably.
For example, hash tables, readtables, packages, streams, and functions might not print readably.
A common notation to use in this circumstance is #<...>. Since #< is not readable by the Lisp reader, the precise format of the text which follows is not important, but a common format to use is that provided by the print-unreadable-object macro.
For information on how the Lisp reader treats this notation, see Section 188.8.131.52 (Sharpsign Less-Than-Sign). For information on how to notate objects that cannot be printed readably, see Section 184.108.40.206 (Sharpsign Dot).