Venice

I, personally, don’t think you need a guided tour of St. Mark’s. We bought the timed-entry combo ticket that allowed us to see the Basilica, the Golden Altar, and the Museum upstairs. We did do the Secret Itineraries Tour of the Doge’s Palace, though, and I highly recommend that. We were on the RS VFR Tour last October and were able to do these before our tour started. We also bought vaporetto passes so we could cruise the lagoon and hop on and hop off at our leisure.

I also don't think that you need a guided tour for St. Mark's. Use Rick's tour from the Venice book. Get the timed ticket with the add ons. You want to go upstairs to the museum to get a closer view of the mosaics on walls and ceilings and to go out onto the loggia. Also the lights come on at 11:30 a.m. so if you can get a late morning ticket that will allow you to see the mosaics in all their glory.

As for other "don't miss," riding on the vaporetti is one of my favorite Venice activities. Any church will have magnificent art, but the Frari Church (also in Rick's book) is pretty spectacular. Otherwise, just wander around in the "back streets" away from the Grand Canal and enjoy a glass of Prosecco at a cafe.

If you are into modern art, the Peggy Guggenheim museum is small (so, it doesn't take a lot of time - Rick's tour suggests an hour) but contains pieces from her friends - Picasso, Kandinsky, Mondrian, Dali, Pollock, Magritte, Chagall, Calder, etc.

Thank you for this great information, I really appreciate it.

An hour is likely to be inadequate for the Guggenheim these days. While it's not huge, there's a lot of art hanging on the walls, and it's popular enough that you can't just blast through the place without bowling other people over. I'd buy the ticket at least a bit in advance to avoid the on-site ticket line and be sure you can get in when you want to.