If you are planning to drive while in Europe, be advised that one-way rental charges may apply, especially when a vehicle is picked up in one country and dropped off in another.  This can be significant and is usually charged at time of pickup, so make sure you inquire about full charges when reserving your vehicle.  If possible, you may want to alter your route to leave your vehicle in the same country.  Your travel agent can help you sort this out in advance to avoid surprised.