Hellobee Boards

What about getting a globe from a thrift store, painting it black or another color, and having people write their messages in paint pens.

What about a few landmark shaped papers, like the Eiffel tower, Empire State building, pyramids etc. for people to write on? You might be able to use a due cut machine at a scrap book store to do it. Prayer flags? You could also do stars, put up a big picture of earth and encourage people to hang their stars around it and for then to write their wishes on shooting stars?

I love the globe idea! Hmmm...

Maybe, get a globe (or map if you'd prefer something you can frame & hang), print out the continents, cut them up almost like a puzzle (larger pieces, not so much small, into however many pieces needed), have your guests write their note, and then you can pin them on the area of the globe/map they go on.

Or you can do something like this:
http://endlesslyinspired.com/2013/07/24/peace-on-earth-globe-tutorial/
where you tape off lo's name/birthdate/age & have your guests sign the painted part kind of like @Arden: suggested!

@MusicaV: Hmm, you could try a scrapbook store for cutouts, like @cole suggested. Or if you know someone that works at an elementary school, they usually have those machines. Or print out and cut them yourself.