Venice · Italy
St Mark's Basilica is smaller than it looks from the square but overwhelming once you're inside: every inch of wall and ceiling covered in gold mosaics, centuries of loot from Constantinople, and that unmistakable smell of old incense and stone. A guided tour gets you past the worst crowds, explains the stories behind the art, and often includes skip-the-line entry plus access to the upper galleries where you can see the church from above and step out onto the balcony overlooking Piazza San Marco. Without a tour you'll still get in, but expect to shuffle slowly in a dense crowd while security hurries you along.
Expect to pay around €25-45 per person for a decent small-group tour with an English-speaking guide; basic skip-the-line tickets alone run €10-20. The best season is late fall through early spring (November to April) when the crowds thin dramatically, though you may trade that for occasional flooding and colder weather. Summer is beautiful but brutally crowded.
Honest tip: pay the few extra euros for the upper level access if it's offered; it's the only place you can properly appreciate the scale and take decent photos without being crushed. Skip the separate paid treasury and pala d'oro unless you're a serious art historian; they're interesting but not worth the time or money for most visitors.