Saint Mark's Basilica in Venice is a stunning architectural masterpiece that houses a treasure trove of mosaics, sculptures, and the famous Pala d’Oro. I'd recommend getting the full ticket, which includes the Basilica, Pala d’Oro, Museo e Loggia Cavalli and the cost is €20. Alternatively, you can just visit the Basilica for €6 or climb to tower for €12.
Saint Mark's Basilica is probably Venice's most iconic landmarks and it would be a pity not to visit it properly, as it's truly marvellous.
If you are a morning person, or someone that wants to enjoy famous places without the crowds, I recommend waking up early. I'll never forget the feeling of being the only one in the square. I'm not even exaggerating, it was just me and this Venice's masterpiece. I'd go there about 6 am, but you'll be safe from the large crowds up until 8/8:30 am depending on the time you visit Venice.