Scrovegni (Arena) Chapel

Technically not a museum but essentially functions as one. Probably the second most important chapel artistically after the Sistine Chapel. You need to sit in a 'decontamination room' for 15 minutes, after which you have 15 minutes to marvel at the inside of the chapel itself. The frescoes are amazing both for their size and scale but also for their importance in western art. Giotto's work paved the way for the styles of the Renaissance, and his frescoes in the Arena chapel (aka Scrovegni chapel) include the first depiction of a kiss in western art since antiquity. Padua is only about an hour from Venice; well worth a day trip.