To my knowledge there isn't a way to book a gondola ride in advance. The traghetto are just a way of crossing the Grand Canal, so will be at most a couple minutes length. They wouldn't exactly cure a 'gondola fix'. If you're looking to see the city from the water (which you should) I'll second and third the vaporetto suggestion. Taking one up (and down) the Grand Canal is a great way to see the city. I enjoyed taking a gondola ride, but it wasn't cheap (though they can be bargained with), and I don't know if it would make my '1 day itinerary'.

My top priorities would be:
- Take a vaporetto on the Grand Canal
- Visit St Marks Square. If you like art at all, I highly recommend St Mark's Basilica and the Doge Palace
- Wander around the city. Its tough to get lost in since you can only go so far until you hit the Grand Canal, plus there are signs around. Or rather I should say its easy to get lost in, but tough to stay lost for long.