Europe from Spain to Amsterdam

You realise that if you hire a car in one country and drop it off in another there is usually a hefty surcharge.
There is nothing to stop you driving it in all those other countries (you have to tell the car hire company), but you need to return it back to the same country you hired it in. This is because it is registered in Spain, with Spanish number plates, and so cannot be hired out in other countries but has to be returned to Spain.

Check into a leased car through the French-based system, such as http://www.autoeurope.com/buyback.cfm and http://europebycarblog.com/.

Sounds like it could be a great trip. Use viamichelin.com to estimate driving times, and remember that those are optimum times without pit stops. Europe is bigger than it looks on a map and if you get off the tollways, roads can be slow. That's also a time when many Europeans are doing the same, so roads could be crowded.