A fantastic alternative to big European airports. On the mainland across the lagoon from Venice itself. You can buy transit passes here covering both the connecting bus to Venice and the Vaparetto boat system within the city (Venice's equivalent to the Metro).
There are two buses to the Piaza De Roma - the entry point to Venice. There is an express bus for about €4 and a local bus for about €2 (or free with the transit pass). Since the local bus takes only slightly longer than the express, and luggage is okay, the express bus seems pointless. The expensive Alaguna boat to Venice also seems totally pointless: Since you need to take a shuttle bus to reach it, you probably would be there already if you had just taken the standard bus.
NEVER buy any transit pass online before you arrive. It's probably a ripoff.
Venice itself is worth a day, but little more. It's kind of like Disneyland: It gets tired fast. There is a Left Luggage office at the airport, so you can leave you luggage there after your flight, tour Venice for the day, then come back for your luggage in the evening.
For cheap lodging, you probably want to stay on the mainland. I found adequate hostel lodging at Camping Jolly, accessible via my transit pass. (See Hostelworld.com)