FlightAtlas.com

Generally speaking you will get the best price for rental cars if you book in advance. Of course you can book using the telephone but why not book now; online. There are many bargins to be found.

Even if you do book online you will probably be able to upgrade your vehicle (assuming there is availability) or insurance when you arrive at your destination.

If you have a multi-part driving licence make sure you take both parts with you.

If you are travelling across many countries it might be worth considering additional cover from a recovery company.

Drive safe, take breaks, don't drive tired.  Don't drink and drive.