The Doctor and his TARDIS can go anywhere in time and space. It’s amazing, the possibilities are infinite, and now he’s offering Amy and Rory the trip of a lifetime, their choice, like a pre-wedding holiday before they get hitched and start married life. How do you choose when you can pick literally any place and time, past or future or another world?
But their wedding is tomorrow, so their destination needs to be romantic, and where could possibly be more romantic than Venice? Canals, Gondolas, people speaking Italian…
They could have done without the vampires and mortal peril though.