We all know how important trees are. They provide shade, oxygen and use up CO2. The more there are the merrier! Why not plant a tree for each of your children? That will automatically reduce their carbon footprint and it'll be fun for them to look back and say "Wow, that tree is also 21! (or 47! or 83!)" It needn't be in your own garden either. There are plenty of places where you can sponsor the planting of a tree. Just be sure it's indigenous so it fits well into the local ecology.