All the monuments of the complex of the Duomo (that is to say the Cathedral, the Cupola, the Baptistry, the Bell tower and the Crypt) and the Opera del Duomo museum will be closed on Sunday, but they will open on Pasquetta day. Anyway, keep in mind that on Easter morning the so called Scoppio del Carro, or explosion of the cart, will take place right in front of the Duomo. It's one of the most important traditional events in town, so consider attending the celebrations.
Last but not least, Villa Bardini with its beautiful garden, and Palazzo Strozzi, boasting the new exhibit Dawn of a Nation. From Guttuso to Fontana and Schifano, will be open both on Easter day and Easter Monday.
Well then, buona Pasqua and buona Pasquetta!