I was in your shoes about 8 years ago, albeit a little younger than you at that point...For me, it was a no-brainer...

Pamplona, Spain in July for the San Fermin festival, AKA The Running of the Bulls.

Craziest party I've ever been a part of. I did have a friend meet me there who happened to be knocking around Europe at the same time, but there were all kinds of solo people there, and I was surprised at the number of females there. I don't remember if I actually ran with the bulls or not, but the party was from sun up to sun up the next day.

I stopped over in London and Paris on my way over (met a different friend in Paris and did London solo).

I did an stopover airline award ticket (into LHR, back and forth from Paris, LHR to BCN and home from BCN) on *A. I did have to buy the ticket on Iberia from BCN to Pamplona, but it all worked out very well.