@Ceili,
I really don't know what to suggest. I've only been to a few places myself, and everywhere I've gone to has its own charms. I took my mate on holiday to Amsterdam last year. That was the first time he'd been, and now he doesn't want to go anywhere else.
Italy is extremely beautiful, if you want to look at really gorgeous classical architecture Florence is incredible. I think it's one of the most beautiful cities on the planet. I've only been to Tuscany so I can't really talk for the rest of Italy.
Germany and the Czech republic are also very beautiful, but in a more gothic way. If you're going in Autumn it will still be quite warm along the Mediterranean coast, and you should be able to go swimming in October.
To be honest I don't really know. There's loads of places I still want to go, Berlin, Vienna Copenhagen. I wouldn't try to get too much in in too short a space of time. You don't want to spend most of your time stuck in a coach.
I'd look at things like exchange rates, and the cost of flights/hotels. Than can make a big difference. You can do a lot more with your money in Prague than Vienna for instance. That's why I've been to Prague and still want to go to Vienna.
I can only talk about what I know which isn't a huge amount, but if you've got any questions about where I've been I'll try to help out.