The cross-on-square symbol means to begin, then stop, rocking backwards in your chair. The 25 symbol is a cue marking when the offstage speakers begin blasting Chicago's "25 or 6 to 4." At the indication 80, each audience member begins singing their favorite 80's song in falsetto. At 89, each player reaches over and removes the hat of the player on his or her left, unless there is no player there in which case the performer would pop a pre-inflated balloon. For the empty grace note, the player may either switch instruments with a neighbor, or call out "nugget!"
Perhaps someone could post the composer and piece title. These instructions may differ from mine.