The St Marks Basilica is in one word beautiful with its Gold Mosiacs, frescos and relief paintings makes this a must see in Venice. Please bear in mind that the lines are always super long to get in, so buying your ticket ahead or using a guided tour might be best. It is also noteworthy that this is still being used as a church, therefore it is not customary to come in with shorts and sleeveless clothes in reverence of it being a holy sanctuary. If you have time visit the terrace and museum above the church (for a few euros) and you’ll get beautiful views of the piazza as well.