I like the idea of the local high school field...
When I used to shoot bows I would take and place a target and then walk away taking choppy, long, and different steps drop an arrow, then do the same to get to another spot...then guestimate the range and shoot it. Just what people wanted to see, a drunk on the local high school field shooting arrows...
I really recommend you don't do this with an AR...
Point being hit the local field take a few baseballs/footballs, toss em out at various distanced and estimate the range, then verify with the yard markers.
Used to do it just walking around too. For example, walking around the mall, pick a store front, kiosk, group of people, guess the range, then pace off one yard per stride. Should get you within a few yards of the true distance...