Renting a car is a good idea if you want to travel around Valencia and visit the more secluded towns in the Valencia region,  but for the city of Valencia itself, public transport is more than sufficient! Renting acar in Spain is relatively cheap, but you have to be aware of the extras such as limited mileage (which means you only get about 100 km per day included in the price after which you are charged extra per km), accident waivers (meaning how much you are liable for in case of an accident), etc…