Just returned from Salou last week and had a hire car for the duration of the holiday. Travelled through to Barcelona twice, one to buy tickets for the Barcelona v Levante game and the second time for the game itself. Car was fine parked and left for a number of hours on both occasions. Driving was okay and roads well sign posted. Toll costs were about €9 between the two places, you can avoid the toll road but it is a much longer journey. We used Auriga Crown Car Rental which was the cheapest we could find however we got charged €52 for a full tank of petrol when it only cost us €30 to refill it. It probably still worked out cheaper than others in the long run and it was a brand new car with only 1800km on the clock. I would not hesitate to hire a car and for us staying in Cap Salou felt it was a necessity as there was not much round about us (altough public transport was very good too.) Hope this helps.