You can buy a day pass @ the visitor center or at the self serve kiosk.
They do have day use areas. There's a pay station just past the visitor's center as you enter the park. If the kiosk isn't manned there's also a machine that will take payment and print a day pass.
There is a check-in station, you can check in there, if nobody is there, you can check in at the kiosk. It's a $9 fee for the day.
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