Handed our 13 year old the cash and the plan for one Sunday. Here's how it went

We keep telling our son he has no idea what things cost, so last Sunday we gave him a fixed amount in cash and asked him to plan the whole day for the three of us: where we go, how we get there, what we eat. Our only condition was being home by 7. He picked the metro over the car to save money, a museum because it was cheap and air-conditioned, and lunch at a place a friend had recommended. He ran out of money by 4 because he hadn't counted the auto from the station in both directions, and had to bargain for one ice cream with what was left. What surprised me was how careful he was. He checked the menu prices before we sat down, which I have never once seen him do with our money. We're planning to make it a monthly thing. If anyone has tried something like this with a teenager, what did you change the second time?