Formalizing a car rental in Valencia is not a problem. You can simply do it at the airport, when you arrive at the destination. There the offices of the companies that give their service in the area are concentrated. The main international companies, all regional and local are present. There are also several offices at the Nord Train Station and at the Bus Station. Also throughout the city you will find many of them and renowned brands.
The cars they have are modern and very new in a wide variety of models. Prices, as in any other city in Europe, range from € 25 a compact to € 125 a luxury one, per day.
The requirements for hiring, the common ones in Europe: driving license, identification in order and, increasingly, require the guarantee of a credit card in the name of the owner of the rental.