In Spain... Benidorm, Alicante', Cadiz...Palma De Mallorca
In France... not to far from Nice... Hyeres. It's a smaller coastal town, a little cheaper and fairly close to Nice and St. Tropez.
Italy: My favorite parts of Italy were the Southern end and Sicily. Yes All the tourist attractions are in the North but if you want to see the best parts, I recommend either Calabria, in the South or some of the Towns up North near the Alps. Or Messina or Taormina on the island of Sicily.
If you want a straight up European summer party spot then you want to go to either Mallorca or Ibiza (more expensive)
I spent 5 years living in Europe(Sicily)To me all the big cities are the same, its the culture you find in the small towns and rural areas that makes all the difference. I mean, by all means, hit the big ticket items like the colliseum, etc, but don't spend all of your time just in the cities...
Rob