For those who live in big cities or the suburbs, consider making a trip to the forest or mountains. I have yet to meet anybody who has gone to national parks and regretted it, and if you come prepared with your own food and sleep in the car, the trip can actually save you money since you're not buying overpriced coffee or meals from foodtrucks for lunch.
One other thing to note, I would actually avoid most parks during the free admission days. The popular parks are absolutely swarming then. Smaller parks far away from major populations can be fine though.
I went to a national park 30 miles outside the path of the solar eclipse, the day before it happened. Would. Not. Recommend.
The eclipse was cool though.
Yup, great value. Passes are great for visiting a ton of areas in one year. Plus with how much prices for parks have increased for a single visit, it only takes 3-4 visits to break even.
I thought about posting the link myself, but I really hate to shill for anything--even if it isn't a company, and even if it genuinely is a good value.
For those living in the USA; September 22 is National Public Lands Day and all parks are free admission.
The annual $80 America the Beautiful National Parks and Federal Recreational Lands Pass allows unlimited entrance to more than 2,000 federal recreation areas, including all national parks that normally charge an entrance fee. There are also free or discounted passes available for senior citizens, current members of the military, families of fourth-grade students, and disabled citizens.
Pass info here