island nearby are much better. If you want to go to St Marks Basilica or the Doge's Palace, or the Rialto Bridge to check the tourist boxes, you can do it in half a day. Any more than a day is way too much. And even that day can be better spent elsewhere.
I have been there multiple times and won't go back unless it's with someone who has never been and wants to check the box. It was the least favorite for my daughter of 6 places in Italy she and I visited last summer.