Have you already booked the car? Bearing in mind all my experience is in Spain, and these are the common scams with Spanish car hire companies.
If not, make sure you get one with a full to full fuel policy unless you know you will use a whole tank of fuel, otherwise you will pay about 100 euro for fuel you don't use.
Decline all extra insurances sold by the rental company as you have already bought the extra insurance. Be prepared to put a 500 to 1000 euro deposit on a credit card.
Don't rent with Goldcar, they are shit.
Don't forget your driving licences and DVLA code, but if you do, you can phone the DVLA there and then and get it sorted over the phone - voice of experience. DP forgot his driving licence and the choice was either I drive or he phone DVLA from Spain. I'm a good confident driver here but I simply cannot cope with driving on the other side of the car - I mirror image everything and get it wrong. It's a lot less stressful for D P to drive all the time on holiday than it is to be a passenger while I fuck it up.
Read the moneysavingexpert guide to car hire, for pretty much everything you need to know.
When you collect the car, check the damage report carefully and photograph any dents and scrapes. This will be invaluable should they try to charge you for damage that you didn't cause.
Having said all the above, DP and I have rented cars in Spain dozens of times and have never been scammed, but have kept our wits about us. This is despite DP crashing our first hire car ever in Spain into the back of a bus 10 minutes outside the airport due to 'wrong side of the car' spatial errors. The car hire company didn't give a shit, didn't charge us and all was fine.