"A little bit of Aggression" is a great way to give a classroom practice with subtraction while engaging your top students in tough problem solving. Think of it as a superior variant of Risk. There are a number of critical differences: 1) Armies do not move and 2) Armies attack into a neighbouring area and win if their total value is greater than the defender. The subtraction enters play in setting up armies.
It could be the only thing that students do in math class for grade 2 or 3, and I guarantee they will do better than most carefully designed curricula. It is really that good.
In 1973 Eric Solomon created it. This pdf adapts the original to the mathematics classroom. It is the greatest educational game that I have encountered (I am a game designer myself - so this is a huge compliment)... "A little bit of Aggression" is strategically deep, cheap, looks nice, and can connect the math classroom to geography and history if you desire.