I love Venice, but half a day is scratching the surface of a very large thing.
As a general strategy, whenever you see a large congested area ahead, basically the entrance to the city in Piazzale Roma/Santa Lucia Station, Rialto Bridge and St Mark's Square, go left or right, and things will quieten very quickly down the side streets/canals. You will find canals that will be calm like mirrors if no one has been down them for awhile.
San Giorgio Maggiore, as mentioned, can give lovely views back to the main part of the city, and St Mark's Square, from ground level, and much higher if you go up the bell tower. You do need to get a Vaporetto there though, and they are not cheap. Unless you want those views, I would stay to the smaller side streets on the main areas and walk. The more you walk the more you'll see.
I was staying in Venice in August, and went to Verona for the day. They were further from each other than I realised.
I only spent a couple of hours in Verona, and didn't see much. I wasn't in the mood to wander too much that day.
Hopefully you will see the parts I missed.
You can see some of my Venice images
here if you are interested. If there are any locations that catch your eye, let me know, and I'll show you where they are.
January will not have much daylight, so if you've not already thought of it, try and get there before sunrise to maximise what light you will have. At this time of the year St Mark's would probably already be quite busy at sunrise, so I wouldn't be rushing to get there, if that was possible. You can get an idea of how busy the main parts of Venice are at the different times of day by looking at the live webcams at various parts of the day before you go. One with the most webcams is
Skyline Webcams, and there are more higher quality webcams on the
I Love You Venice YT channel.
I hope you get a bright/sunny day to maximise what light you'll get, as it is all very narrow.