Ah, I love Paris! That's great that you will be going, although I'm sure it will be tough to be so close to Madrid but not able to visit. I don't know if you read
this thread already, but there were a few posts last fall about Paris that might give you some ideas. I love just walking around, window shopping, enjoying the fabulous cuisine and people watching, too.
One suggestion before you go would be to watch the movie "
Paris Je T\'Aime ." I saw it not too long ago, and I really enjoyed it! It's 18 short films (each 5 minutes long) about love in Paris, and they are all directed by different famous international directors with famous international film stars.
Bon voyage!