First of all try to change your hotel if you can. At the very least plan on spending the late evening in Venice as it is truly a whole new world when the tourists leave, the sun sets and the street lamps come on – magic!
For one day I would not make any reservations for anything and instead just spend time wandering and relaxing and soaking up the city atmosphere. Here is how I might arrange my day:
From Airport take Alilaguna ferry to Piazza San Marco (Euro 12 pp) – takes about 70 minutes so get on earliest one you can!
Arrive Piazza San marco – walk around – go inside Basilica and marvel at gold mosaics – if line’s not to long and weather is good, go up Campanile (bell tower ) for marvelous views.
Wander up Riva degli Schiavoni (along the waterfront) and admire the views, crowds of people and turn nose up at all the trinket stands (you can do better for souvenirs…skip these.) Go quite far down until crowd thins – take a table at one of the waterfront cafes FAR down the way and split a slightly overpriced pizza or salad and bottle of wine (better service, quality and price the farther down you go….). At least stop at a cafe and get an espresso or bottle of water or glass of wine – stand up at counter for cheapest prices.
After refreshing snack, get on vaporetto (water bus) at Arsenale or San Zaccaria (depends on how far down you are) and take Grand Canal cruise up to Rialto Bridge – get seat or stand at back for best views back down grand canal or get seat or stand at very front for best views of Rialto bridge.
Exit Rialto Bridge and wander across with the incredible hoards of tourists doing the same thing – good photo op from top of bridge and down canal – continue over bridge and through the mass of souvenir stalls and get to the outdoor markets – if you like seafood keep wandering a bit further up this main drag to the incredible outdoor fish market – quite a sight!
Turn back around and go back to the Ruga Rialto and turn right – wander this great street through the fantastic San Polo district, marvelling at how few people there seems to be here until the street narrows and it suddenly becomes insanely crowded for 30 yards – where did all these people come from? – If you didn’t eat earlier now is a perfect opportunity to pick nearly any trattoria, cafe, or stand up pizza stall (Antico Forno is exceptional) for a snack – IF you get to Campo San Polo, go to the far end of the campo and to the right you’ll see Antica La Corte Birreria (great local place for pizza, salads, light meal and dessert – reasonable price and good food!) You will find many good and interesting shops along this street and the side streets – I recommend shopping here if you want souvenirs or Murano glass trinkets or jewelry.
After a refresher and resting your feet, continue up a bit futher to the Chiesa dei Frari and take a few moments to go in and revel at the stunning art and sculpture and tombs in this hidden church – Titian’s Assumption at the main altar glows red and gold from across the room and there’s a stunning Bellini triptych – see Canova’s dark and morbid pyramid tomb across from Titian’s opulent one.
Exit into the sunlight and go further up to Campo Santa Margherita – very active place with lots of bars and small pubs and shops. Alternatively, if your feet hurt, go to the nearby San Toma vaporetto stop from the Frari church and get on the vaporetto, exiting at the Accademia stop.
Either way, wander to the Dorsoduro area and find characteristic shops, etc. here also. If you enjoy fine art, you might check out the line at the Accademia – in the evenings it tends to be light so enter if you like. The sun should be setting soon after your departure.
Take the traghetto (a gondola used solely for the purpose of ferrying people directly from one side of the canal to the other) for 50 cents from Dorsoduro and back to San Marco. Enjoy the sunset – climb the campanile bell tower now for great views and the sunset, if you didn’t do it earlier.
If you are done with your day and don’t wish to have dinner or stay the night, take the Alilaguna home from here. Otherwise, find a nice trattoria or ciccheti bar (like a tapas bar) and rest your feet again. End your evening with an overpriced drink back at Piazza San Marco, enjoying the music and mood under the moon in the piazza. Then you will need to take a waterbus all the way back up the canal to Piazzale Roma and take a taxi or bus back to your airport hotel. The Alilaguna probably will have ceased running at this time of night, but check their schedule so you’ll know your options.
A busy but satisfying day to be sure and it takes you in a circle loop around Venice that will give you a good overview of the city and what it has to offer. I suspect you will want to come back again some day and see more. Get a good map and follow this route I suggest and you will see how easy it is. Get a one day vaporetto pass if you will take 2 trips or more by waterbus. If not, then a one trip ticket will be ok. I think they also offer a 12 hour pass, but you should check on that.