Venice museum tickets

St. Marks for sure, else you'll be looking at a long line of people buying tickets. The line moves fast, but I spent a solid 30 minutes in that line in September 2022, and there are more people traveling this year. The good news about St. Marks is that it is comparatively cheap.

The two times I walked past the Guggenheim there was a bit of a line. I assumed it was for people wanting to buy tickets, so I went online and bought a ticket for a few days later. I don't think the Guggenheim sells out far ahead of time, but it is popular. While it's not as packed as the Vatican Museums in Rome, the conditions were quite crowded in 2022 by art-museum standards. I still really enjoyed it, and I'm planning to go back later this year.

I'd be very surprised if there's any issue at the Fortuny or Ca' Pesaro.

I know the Secret Itineraries Tour at the Doge's Palace is popular. It definitely sells out in advance; I don't know how far in advance, though.

There's also the rooftop viewing platform at Fondaco dei Tedeschi. It's free but requires a timed ticket, and I think I've read that those get snapped up immediately when they become available. https://www.dfs.com/en/venice/service/rooftop-terrace

I can tell you that the Tedeschi tickets come out 21 days in advance and are grabbed almost immediately. We were unable to get them 21 days in advance. We tried tickets for daytime St. Marks a few weeks in advance and also could not get them. Someone recommended put in museum with it as well and it might work. That did not help. We ended up booking a tour by night. Decide what you absolutely must see and buy the tickets you want is my advice.

Thank you both very much for the advice!