Watch a sunset
Pick a nice spot from which you can watch a sunset (we'll call this point A). Ideally, you'd have a clear horizon in front of you, and behind you would be some sort of elevated point that you can quickly access (a hill, a building with at least two floors, or perhaps the aforementioned tree; we'll call this point B).
Watch the sunset from point A, and once the sun is out of sight, hurry on over to point B. With the added elevation provided by point B, you should be able to see the sun above the horizon. If Earth were flat, the sun would not be visible at any elevation once it had set. Because Earth is round, the sun will come back into your line of sight.
