We've also plumped for the Greek Islands for three weeks.
We're going to go to Crete for a week - staying near Rethymos by the beach. Lots of culture there (The Palace of Knosso) and good food. Then off to Athens for four days - we've found a 5 star hotel with a rooftop pool for only 100 EUR/night (The Classical Athens Imperial in case anyone is interested... No shortage of culture there!). Then we island hop for the final week to Mykonos and also Santorini for some sun, sand and relaxation.
Some other ideas that might suit you:
Turkey - Istanbul + then down to the coast for some sea and sand?
Italy - the Italian lakes are beautiful. The city of Verona is stunning, and would be a perfect spot for some culture (the open air Opera there is very famous). You could then jump on a high speed train down from Milan to Rome and visit the Amalfi coast
France - What about Bordeaux? This is one my favourite French cities, where you can enjoy the beach (go to Arcachon enjoy the beautiful sands), slurp on fresh oysters, tour the stunning vineyards. I love the city of Bordeaux also... I need to go back! (H2B studied there before we met and he has infected me with his love of the place).
Portugal - Lisbon + the Algarve. We went there last spring with a villa from villaplus.co.uk to the town of Guia, and there is plenty to see and do. There are so many beaches, it is easy to find one that isn't too crowded. I love it there. Lisbon is an awesome capital city as also, and a short flight away.
Switzerland - It's very expensive here, but I highly recommend Zurich for a city break. There is a stunning lake to go swimming in, and plenty of nightlife/great restaurants/luxury. You could use this as a stopover if you were to then jump on a train down to Italy.
Croatia - Split & Dubrovnik are reportedly excellent - I've yet to go, but could be worth investigating.
Will marry on July 9th 2011 in Geneva, Switzerland.