Senoirs can purchase a discount card that is good for free admission to all national parks. I have been using one for years. I think the price has increased, but still a bargain if you visit the parks often. https://store.usgs.gov/senior-pass
I'm gonna go way out on a limb and suggest asking for the senior rate when you purchase your tickets. It's so crazy it just might work!
Many National Parks visitor centers sell the pass. You should be able to purchase at the park visitor center.
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