On a budget, St Mark's Clock Tower (Torre dell'Orologio) still earns its price.
~€14 (guided, booking required)
A guided-only climb takes you inside the workings of the 1499 blue-and-gold astronomical clock, up past the mechanism to the rooftop where the bronze 'Moors' strike the bell beside a close-up of the winged lion — a small, genuinely fascinating engineering visit. The catch: it's by advance booking only, in small timed groups, the stairs are steep and tight (not for the claustrophobic), and from below the clock is free to admire anyway.