Failing to plan your visit to the park
You wouldn’t leave for New York City with no idea of where you’re staying, what you're going to do, or even how long it's gonna take to get there, would you? Of course, not. Yet amazingly, some fly-by-the-seat-of-their-pants tourists don’t plan any of their trip to the Grand Canyon and are so overwhelmed when they arrive that the day pretty much begins and ends at the visitors’ center. Be smart and do a little online recon before you go. Research the hiking trails and find one that matches your ability, rather than just barreling down the first one you see.
Trying to do waaaaay too much
On the flip side, some people try to hike to the bottom, see Havasu Falls, and drive to Hermit’s Rest all in an afternoon. Slow down there, camper. The canyon is 277 miles long, 18 miles wide, and 6,000ft deep, and you could live in it your entire life and never see everything. To give you even more perspective, you could put every person on Earth in the canyon, and still not fill it up. Pick one thing to do per day, maybe two if you’re feeling ambitious. But just accept that you can’t do it all.