Formal structure - the use of balance, symmetry and varied repetition - is one essential characteristic of "classical" classical music, as in the work of Haydn and Mozart.
Use a picture of a classical building - the Parthenon would be a good example - as a stimulus for an instrumental composition and as a graphic score to inspire a pattern of regularly ordered sound.
Look at the balance between the vertical and horizontal components, and try to find an aural equivalent. Perhaps the orderly arrangement of the columns makes you think of a repeated rhythmical shape. Maybe the entablature used could turn into a calm stately melody graced with small decorative inventions.
Use regular patterns of letters to help explore the shape of a classical minuet and trio. This very often takes the form AABA, where A represents one tune and B is a contrasting section sandwiched between repetitions. In groups, compose a piece using the model of AABA, with different arrangements of instruments marking the transition between sections.
Listen to 20th century "neo-classical" music, such as the first movement of Stravinsky's Octet or a section from his Pulcinella. Can you hear how the beautiful tune of the Octet has the shape of a Mozart melody, rounded off with a deliberate rhythmical stumble? What does he do with harmony and instrumentation in Pulcinella to make the elegant classical tunes sound edgy and modern?