When I went, I visited Doge's Palace which was pretty amazing, also you can access the Bridge of Sighs from there. Obviously, the Gondolas and Piazza San Marco and Saint Mark's Basilica is a must. Also, Venice is famous for glass blowing and there are factories where you can go and watch them make sculptures​, can't remember exactly which one I went to but it's worth seeing. Hope this helps, If you have any more questions just let me know :)