We went to Venice in March and the weather was lovely. Before travelling I was concerned about the canals smelling, but the water was constantly pumped around so it was only at night that the smaller canals in the back alleys were a little smelly! This however was not noticeable in the main tourist areas! St Mark's Square and feeding the pigeons are a must and I would advise you to stay in or around that area, as this is where most of the attractions are. The bell tower in St Marks square is a must as the views of the city are fabulous. Also the Rialto Markets are a must do - just off of the Rialto Bridge.
Quick Tips:
I would say not to eat on St Mark's Square as the restaurant prices are extremly high. If you walk 100 metres down the road from there you can eat for half the price. The restaurants near the Rialto Bridge on the Grand Canal were much better prices and the views are better as well. Also, Venice closes down early in the evenings (about 11pm), so make sure you don't leave it too late to eat. Also the better restaurants in Venice may require you to book a table. I would suggest you to buy a guide book to plan what you would like to see before you go, as it is bigger than you would think!Best Way To Get Around:
There are only two ways to get around in Venice, on foot or by boat. The vaparetto is like the local bus service very cheap (about 50p for one way journeys), the water taxis are expensive and also will charge extra for luggage. If you are nearer to St Mark's Square, it is easier to walk, and it will take about 15 minutes to get to the Rialto Bridge, but be warned, you will need to follow thw green sign carefully or use a map, as it is very easy to get lost, every alley looks the same! Although there are lots of hidden squares to explore!