I'm not going to take my 13yo son to Magic Mountain this weekend
Is there a reason why you chose not to?
Because I can't afford it right now.
I'm sure your son will be disappointed, but explain to him that you can't afford the trip right now. Make him understand that sometimes you just don't have the money for extras after paying for necessary items. Tell him you'll make up for the missed trip in the future.
I'm going to take him next week.
See, there you go! It is okay to not always be able to afford the extravagant trips. It is kind that you want to make up for it next week though.