Sixt car rental is very easy to rent. Just book your car online days before the rental day.
They will offer you a lot of extras like insurance, wheel coverage, fuel coverage etc but we had international insurance (which is a must to get) so we were fully covered. Just remember not to get the extras as it cost double the rental price!
Also, before returning the car fuel up with full tank. As they charge high for the fuel.
Lastly get a diesel car and automatic. That is our best decision!
Another tip the store is for booking and paying (via credit card only) you will need to get the car on the basement of plaza de España, they dont have lifts so be prepared to carry your luggages.