Organize a charity run!
The students run a previously marked-out route. Every one runs as many rounds as he/she manages. For every kilometer they get a certain amount of money from their sponsors. It is satisfying for pupils to see how much money they can raise through their personal effort and they are proud to donate the money as a school team to a charity.
A beautiful slide presentation specifically for students (and teachers) can be downloaded here:
The presentation can be well integrated into lessons of geography, religious education, free lessons etc. It is also a good introduction into the topic of development aid and gets students generally really interested in the matter.