The Fondaco dei Tedeschi is a historic building in Venice, located right next to the iconic Rialto Bridge. Nowadays, it houses a shopping centre.
Aside from the shopping aspect, they have a stunning rooftop opened to the public for free, and it offers one of the best views in the city. To visit the terrace you have to book in advance, just use the link I put here, and make sure to go there at the time indicated, otherwise they might not let you go up.
The terrace is open every day from 10:15 am to 6 pm.