On my course there is a spot after hole 10 where you cross a road and there is a stop sign near it. A lot of locals play this rule for fun. Hit the sign from the bush before you cross the road and you can drop a stroke. You get three tries and can only drop a total of one stroke (so people can drop more than one).
On a friends course there is a made up 22nd hole using the practice basket and teeing off close the end of the last hole. When looking at it you would be surprised at how much it is like an actual hole and not a practice spot as most practice baskets are out in the open next to a board or parking lot.
Any other fun ones?