The Cathedral of Monza, together with the chapel of Teodolinda and the Museum, is a small gem to discover (and rediscover) in the heart of Lombardy. Exhibition rich in late-antique and Lombard artifacts (if you studied art history, half of the illustrations of your book will see them live here), the small museum of the Cathedral is worth a visit, given the overall negligible cost to access it.
Different speech for the Chapel of Theodolinda, which requires mandatory reservation. If you want to do it directly on the spot, we recommend that you aim for the less frequented times, so as to find free places. In any case, it is a destination that I strongly recommend and that deserves, alone, the visit of Monza. It is a 360 degrees immersion in the great art of the fifteenth century and in the history of the duchy of Milan, of which Monza was one of the protagonists.