Don’t worry, I do it too. Try to think of something fun to do while ignoring the possibilities inherent in something I’ve already paid good money for, such as zoo or amusement park memberships.
This is free in the sense that you’ve already long since paid for the membership. If you got the right kind of membership you won’t even have to pay for parking.
Going to one of these places can mean a lot of temptation to spend money, at the very least on lunch. But many places allow you to bring lunch, or you can take advantage of the fact that you have a membership and just plan a shorter day there. If you can come back anytime you really don’t have to spend the whole day there every single time you visit, do you?