A hundred years ago a group of us from Fordham brought a quart of gasoline to burn an F into midfield of St. Johns FB field where the teams would play. We did so with the aid of two NYPD cops who thought the idea cool and they drove by distracting campus security. The flame was not great, quart didnt go far but there was a barely noticeable F burned in the field
A great practical joke is to drop turnip seeds on the football field.