For available datasets, see Atlas
Geography being an import part of most school curricula there are a few Open Content projects that could be implemented. There already exist two open content projects that could be adapted for the OLPC.
Openstreetmap.org is a project that is creating a GoogleMaps like online map of the world. But OSM contains data that is Open Content, unlike proprietary versions from Yahoo, Google and Microsoft. However, their editor is in non-free Flash, or non-free java. Gnash isn't supported very well.