Using a window as a dry-erase surface
You can write on glass with a dry-erase marker and it will come off with a paper towel. (Try it yourself if you don't believe me!)  I've never left it on glass long enough for it to need any sort of solvent to remove, but I'd think that normal ammonia-based window cleaner would work OK; if not, you could always use dry-erase cleaner as a last resort.

The reason this works is that dry-erase boards (the rigid kind at least) have a thin ceramic coating to prevent the ink from penetrating pores in the white plastic substrate, and glass is a ceramic.

If there are any stubborn dry erase marker smudges left over, a sharp #9 razor blade will easily remove them.