I second the suggestion of going to Ostia Antica while in Rome. It’s easy to get to on public transit, and is often overlooked by most tourists so will not be crowded. It’s an ancient harbour town that is quite well preserved, and you can walk all over it and into the buildings.
Venice needs a minimum of three days. It is fairly easy to walk to the main attractions, but it is also easy to get lost in all the narrow alleys because it’s hard to see where you are going. Google maps on your phone will save you, and you will see all the tourists running around with their phones out to guide them.
The vaporetto is expensive. A single ticket is 7.50 euros and is only valid for 75 minutes. You can buy a 24 hour pass for 20 euros, and there are options for 48 and 72 hours also. If you want to get out to some of the islands, which I highly recommend, I would suggest that on one of your days you purchase a 24 hour pass and do all of your vaporetto travel on that day. Get the vaporetto from the Fonte Nove stop and it is only a six minute ride to Murano and all the glass shops. Spend a couple hours there, and then get back on the vaporetto to continue on to Burano which is famous for lace making. If you aren’t interested in lace making, still make the trip to Burano as it is the most picturesque of the islands with its multi-coloured buildings. You could also make a stop at nearby Torcello.
Now, since you have a vaporetto pass, get on the route that goes down the grand canal ( line 1 or 2) and just ride the whole canal to see all of the buildings from the water. I would also recommend doing the same trip after sunset when all the building lights come on. Otherwise, St. Marks square is the main tourist hub, and this is where the basilica is, as well as the Doges Palace, and the Corer museum. You might want to look into the various city passes that are available to save money on entrance and not have to stand in line to buy tickets for each one.
The best photo ops are on the bridges that cross the grand canal like the Academia bridge, and the famous Rialto bridge. These get quite crowded, but it’s worth it to get up early, at sunrise, and get out to get photos before the crowds show up. There is a tower at St Marks square that you can pay to go up to the top and get sweeping views of the city. But, if you look across the water you will see another tower at the church San Giorgio Maggiore, which you can also go up, and it’s cheaper, and likely won’t have a line up( or a much shorter one). Do this on the day you have a vaporetto pass.Other popular places are the Acedemia Gallery (old masters, mostly religious themed art), Peggy Guggenheim gallery(modern art), and the Frari church. There is a variety of other museums, and churches you can visit by either paying as you go, or buying one of the passes. If you are interested you can learn to navigate a gondola (not sure of the price), or just take a gondola ride (expensive at about 80 euros for half an hour). Otherwise, the most fun can be had just wandering and getting lost.