How to clean white garden stones?

hi there, i have a small courtyard that is covered in white garden stones, however they've gone a bit grim. we're renting but moving shortly so need/want to get them to a reasonable standard before we move. any ideas on the best/cheapest/efficient way to get them looking half decent. i really don't want to have to pick them all up to clean them. i had thought of a pressure washer but i'm told that this would not be wise as the stones would fly up and hit the gl doors (and me!). any help would be much appreciated. thanks in advance