Fatto generale: 688 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 688 is an unremarkable number.
Trivia: 688 is an uninteresting number.
Data storica: November 18th is the day in 1730 that Frederick II (known as Frederick the Great), King of Prussia, is granted a royal pardon and released from confinement.