I'm going to Barcelona in just over a week, and it looks like it would suit. Though, the city doesn't come 'alive' until the late (or early, rather) hours. We're paying about £130 for flights each, with Ryanair but booked quite late.
This probably won't suit, as you said you wanted south (warm) but I just looked into Amsterdam for the end of August. It's an awesome city (with so much more to do beyond the obvious) and perfect for a couples holiday I reckon. £100 pp flights with Easyjet which is a bargain.
Honorary mention for Berlin, because it's awesome.