I find the best thing to do before going on a weekend away to for example: Paris, is to spend lots of time researching the place.  Buy one book from Waterstones, AA Spiral Guide for instance, and buy a notebook from Partners.  Write in the note book on seperate places where you would like to visit, the metro stations/bus stops near, prices for entry etc, and research more on the internet!

You can't go wrong. You'l have a notebook easy to carry around with you.

Also remember a map.. and stick it in the notebook. It could save your holiday!