Europe in the Spring

I'm seriously considering the idea of creating a tradition of taking annual European trips every spring. I was just looking at round trip tickets to different cities in Europe and they were all around $600. I've always heard that travel to Europe is cheaper in the spring, but I've never considered going because I always thought a week was too short of a time to travel so many miles at such an expense. However, I'm starting to tweak my perspective to include the realization that spring break is really 5 week days + two weekends; 9 days together with an addition of two personal days every year could give me 11 days to choose one European city and have at it. Think of the adventure! Holy cow, this could add the excitement my poor little adventure starved heart craves! So here is an invitation: if you're interested: next year I'm thinking of London or Oslo.