About Venice:
You should visit Piazza San Marco(Mark's Square), where stands a beautiful cathedral and you can also find Palazzo Ducale(Ducal Palace). I'm afraid you will have to pay to go inside, but frankly, an external view it's enough
You could also enjoy a ride on a steamboat that goes through the canals, which is quite suggestive, or just walk across the streets(and it's really easy to meet blind alleys), and the bridges, some of them are cool and offer some nice views.
The museum list is really long, one above all, the Peggy Guggenheim collection.
Going there in March means that you can find the Venice Carnival, which is amazing, but I think it will be at the start of the month.
Last thing for now, Venice by night is very beautiful and romantic, perfect for couples, so it's worth spending an evening there, with a good aperitif