For me, one of the perks of being on an airplane (aside from the fact that it's taking me somewhere awesome) is that I have extra time to read. Especially on international flights. In the past I've breezed through two or three books on 6+ hour long flights. Honestly, what else are you supposed to do with all that time? I have trouble sleeping more than thirty minutes at a time on a plane, and the in-flight entertainment isn't always all that entertaining, so I always make sure to bring a few books with me. Not that I need an excuse to read, but when I'm entirely disconnected from the internet, social media, and everything else going on in the world, books seem like the obvious choice.
The flight to Madrid is 7.5 hours long, and the flight back is just over 8 hours, so I'll have plenty of time to read. Add to that any down time I'll have while I'm there, and five books doesn't sound too difficult to get through... I'm going to download the kindle edition of most of these, just to ease down on the bulk of what I'll be traveling with.
#1 The Hitchhiker's Guide to the Galaxy by Douglas Adams
#2 The Quantum Universe by Brian Cox & Jeff Forshaw
#3 The New Ghost by Robert Hunter
#4 The President's Hat by Antoine Laurain
#5 The Life-Changing Magic of Tidying Up by Marie Kondo