Day 3! I usually have too many ideas in my head to implement, so feel free to steal some.
My theme for today is forest.

1. make a game where you are a forest ghost protecting the woods from human destruction
2. hollow out a log and put speakers inside playing nature sounds
3. create a guerilla initiative for keeping the woods clean of rubbish
4. a project where you buy ruins in cities, demolish them and turn the area into a city forest (alternatively do this the guerilla way)
5. create a free game which emotionally connects citizen back to forests
6. paint street lanterns, so they look like birch trees
7. build a watch tower in the woods, which looks like a hollow tree
8. decrease meat consumption, so less area is used to grow food (especially for pigs are large areas needed to grow them food) - turn the areas free again into forests
9. paint on a dead tree with bioluminiscent paint
10. plant domestic saplings at places currently turning back into a forest / in commercial forests

licensed as CC-BY 3.0 (attribute me as Rick Hoppmann - www.tinyworlds.org
The image is not included in the license.