"Marble offers several Map Themes already - some of them are based on OpenStreetMap Tile data. Many of them can be downloaded using KDE's Get Hot New Stuff feature. Have a look at the existing ones (Like "OpenStreetMap" (using Mapnik as well as OsmaRender), the "Hike and Bike" map, "OpenCycleMap", etc. Have a look at how map themes can be created using the wizard as well as by editing the dgml (xml) files.

Have a look at http://wiki.openstreetmap.org/wiki/DE:Anwender#Online.2C_also_mit_Internetverbindung
which lists a few services. However note that most of these specific ones are integrated with Marble already. So ask the Marble team beforehand whether the map you'd like to work on is already in the works or whether it does exist already.

Create a new map based on such a service (the license of the whole work including the data needs to be compatible with Marble's license requirements, e.g. CC by SA is fine). The new map needs to display the new map material and needs to have a proper icon (in the style of existing ones) including a name. The new map theme also needs to have a proper legend describing all the important colors and icons. The finished map theme needs to be ready for upload to the KDE GHNS service.
Expected results: A new functional map theme based on OpenStreetMap data.
Prerequisites: Knowledge of editing xml files, basic knowledge of Krita, Gimp or Photoshop for the legend."