contributing to a puzzle
Let me recommend Wikipedia, which i recently discovered, not only as a great source of information, but also as a satisfying activity adding to. Writing an article for Wikipedia is the virtual equivalent of helping to build, why not, the cathedral of Reims, or the pyramid of Giza. It's contributing a humble little piece to a puzzle too large to comprehend in full...
The idea behind Wikipedia remains hard to believe: an online encyclopedia in all major languages of the world created entirely by volunteers, in fact by anyone who feels like adding to it. Just go to wikipedia.org and click 'edit this page'.
And the strange thing is, this has not resulted in a total mess, but has actually grown into a full-blown encyclopedia that is now considered to match the Encyclopaedia Britannica (which is so old and revered it's spelled with a special 'ae' character i wouldn't even know how to produce here - plus you need to sign up to use it (the encyclopedia, not the character)).
As an example of Wikipedia's strength: the first little thing i wrote, where i forgot some template and category stuff, was mended by someone else - a random other contributor - within two hours after i had submitted my text.