I agree just wander, eat, shop, etc. The Palace and St Marks are good but not the best in Europe. Murano glass is popular but it wasn't a must for me. As Rick mentions in his book there is a vaparetto boat (public bus) you can take up the grand canal which will give you a pretty good overview. I got the three day pass an road it all over taking the less popular routes to see some of the outer areas.
Rialto bridge is also popular but instead of making it a destination I would just see it when you wander by. An evening gondola ride through one of the side streets is nice as is an evening walk. Venice is pretty safe and has a whole other feel at night.