Last weekend, I took the boys to the big Air and Space Museum annex, out by Dulles. Like all the Smithsonian museums, admission is free, but they charge $12 for parking. After some agonizing, I signed up for an annual parking permit. I'm something of a sucker for unlimited admission passes -- we also got season passes for Six Flags. I'm not sure they always make economic sense, but I'm a more relaxed and happier parent when I know that we can leave as soon as the boys start to fade, without having to endure the "death march of fun" in order to wring out the most value from our admission. Economic logic says that once you've paid the admission it's a sunk cost, and thus the price of admission shouldn't affect how long you choose to stay, but I don't know anyone who actually behaves that way. (I'm also pretty sure that we're still not mentally accounting for the price of gas when when we decide to go to the museum because it's "free.")
The Six Flags passes are an interesting case, because they cost barely more than a single admission. As far as I can tell, they're a loss leader to get you to buy food and rent lockers at the park.