Fuel is the scam. Make sure to note any dents / photograph them as well.
When we rented a Honda Civic last Sept in Crete (the same model will be in Spain I'm sure), we were told it was a better deal to take a full tank and return the car with nothing in it (because Avis supposedly give a good deal for a full tank, which appeared to be the case by our calculations).
So...what we didn't know was that, once the level hits the red zone / warning light it stays on until you put a *lot* of fuel in, whereupon it jumps up. As you can imagine, we were about 50Km from the airport and the needle hit red. We thought another 10 Euros would get us there with any empty tank... 10 Euros later, the light's still on...needle's not budged. We're thinking there's a problem, and we'll miss our plane if we run out of fuel, so we put another 10 Euros in... No movement... After another 10 or so the needle jumps up and we return car with over a quarter of a tank, having opted to pay for a full tank... They said to us 'you must return the car empty, or the deal isn't so good'. I wonder if they program the car to behave this way? They disabled the Ipod dock...that's extra... Avis probably make millions a yr from this 'idiosyncrasy'. We paid for a cheaper car and were thrilled to get the Civic...with its 1.6 Vti engine. There was a reason why you get that car!