If you can manage it, be sure to head to the National Park System on Monday, when the gates will all be open with no need to pay an entrance fee.
Monday happens to be the 98th birthday of the National Park Service, so you just might find some birthday cake in your park travels! There are just two more entrance-fee-free days this year: September 27, to mark National Public Lands Day, and November 11, Veterans Day.
Of course, most units of the park system don't charge fees; just 133 of the 401 units have entrance fees.
While Monday is the Park Service's birthday, and the reason entrance fees are being waived, all other fees for things such as camping, boating permits, and interpretive programs will remain in place.