Despite the term "classical" implies something old, solemn, rather strict or serious, what is called as Classical Music is fraught with humour in all possible forms (strictly musical, as in Mozart's "A Musical Joke", in terms of making fun, as in most of Rossini's output, in multiple forms, as in plenty of Haydn's music, etc.).
The most interesting aspect of this subject is what I discovered in many years of listening experience and study, namely the "concealed" humour in every unsuspected piece or work of classical music. Take in random almost any Scherzo from the early String Quartets of Beethoven and there you are. Observe the musical game and fun in most of the Codas in Beethoven's output and you will be entertainingly amazed. The finale of the Fifth constitutes one of the most brilliant examples of how a brilliant composer can overturn a very solemn c-minor work to a triumphant game of an astonishing use of all the possible means he has and knows how to command and to surprise (just notice the role of piccolo and you will see). Even the fact that he uses the coda as a kind of second development is something to make you smile.
Almost any composer I can think of has enough evidence of exercing his humour in his production of works, perhaps with the exception of the otherwise great Bruckner, who unfortunately failed in many fields (not only in lacking humour) of expressing the different aspects of Life.
It's also intriguing to explore and examine whether one may find anything humorous in atonal, serial, contemporary, etc. music, where I almost fail to detect.
So, folks, do you wish to share your experiences, revelations and discoveries in tracing some good humour in your listening repertory or are you interested in finding what others might find humorous in an outwardly serious, solemn or very abstract piece of classical music? You are most welcome!