Get Closer
This is a game based on David Cox's nice percent estimation sketch: http://www.geogebratube.org/material/show/id/21629
Using it in class students immediately turned it into a game. But it was tough because the 2nd person always got to use the helpful markings.
So I made it into a game! David's sketch is - of course - perfect practice or warm up for the game.
More GeoGebra at mathhombre.blogspot.com