I was fortunate enough to win a competition last year. My prize was a champagne lunch for 2 in any city in Europe with flights and transfers included. Although it was expected that we went there and back the same day, because I live in the sticks the company allowed me to have my flights 2 and a half weeks apart. I went to Helsinki and had my lunch at The Seahorse. Where would you go? I warn you it's not as easy as you think. Mr thugulike and I argued for 6 months before we decided where to go.
Difficult one! If it was just for the day I would choose somewhere like Dusseldorf - it looks a nice place, not too far and unlike a larger city the airport is not too far away. For longer I'd choose er, er, er, Berlin I think, as it's somewhere I haven't been (yet!).
I'm surprised you chose Helsinki for lunch because the flying time is quite long and Finland is 2 hours ahead of the UK.
To avoid this and to choose somewhere warm so that you can sit outside on a restaurant terrace with a view, I think that I would choose somewhere in Portugal, perhaps Lisbon or Porto.
I chose Helsinki because I wanted to travel around the Baltic States and the airfare from the north of England is expensive. As it was I flew to Helsinki with my prize, paid for 3 nights accommodation myself then got the ferry over to Tallinn and went on my trip, returning 2 weeks later to fly home. I wanted to see Helsinki anyway. It was a win win for me.
I've already been to most of the European capitals and wanted to go somewhere I hadn't been before. Venice scares me, I think it's a reaction to the film Don't Look Now.