Hi fruitsalad, I would recommend Venice!! there is nowhere on earth like it, a very beautiful and romantic place, in October the weather is still sunny and warm. We recently went for our anniversary and were blown away by it's "special" feel. I would also recommend the Hotel Ca D" Sagredo, right on the Grande Canal ... it'll be 5* all the way.
We live in Italy, down south, so I would also (naturally!) recommend the Amalfi Coast, Ravello or the Island of Capri, all are just as magical as Venice but warmer as you would be further down South. Your nearest airport is Naples. All of October this year was lovely here, hot and sunny days still great for a spot of swimming and sunbathing without the hordes of tourists but everything will still be open, just a nicer way to explore. Could go on but I know there is a whole world out there, good luck with whatever you decide and congratulations on reaching 25 years! Incidently my in-laws are coming out to see next April and they celebrate they're 39th so we've booked Venice as a surprise for them.