You can take the metro from Rome out to Ostia Antica if you are at all interested in ancient Rome: it is a ruined Roman port but even though it is two thousand years old, it is quite intact. Unlike Pompei, it isn't crowded at all and you get that feeling of stepping into another time and place.
I visited Venice two years ago and was blown away! This was just at the tail end of covid, so it wasn't as crowded and that perhaps added to my experience. But here's a quick tip: there's sort of a main route that runs through the city which 90% of the tourists follow. Don't go that way! When you enter the city, take the first bridge across the grand canal on your right hand side and make your way through the city by the southern route instead of the northern.
The city is beautiful and you'll be able to enjoy it without fighting your way through heaving crowds.
You should still absolutely go to the piazza San Marco and see Saint Mark's Basilica which is a wonder of the wold in my opinion - but maybe get there either early or late so the crowds aren't as dense.
Also consider seeing the Scuola Grande di Rocco where you will find huge paintings by Tintoretto. Even if you aren't interested in art, they really are a sight to behold and the museum isn't crowded.
This should be your approximate route, have a great trip!
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Ostia Antica near Rome
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Paradise by Tintoretto in the Scuola
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Saint Mark's Cathedral in Venice
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