The most open type of
public space, the quintessential public forum, includes places
like ‘streets and parks which ‘‘have immemorially been held in
trust for the use of the public and…have been used for purposes
of assembly, communicating thoughts between citizens, and
discussing public questions’’’ (460 US 37, p. 45; quoting 307 US
496, p. 515). These places are, and must always be, open and
accessible to all, with expressive activity limited only in very narrow cases