You really need to get a little dirt turned up to have the plot that you want. But keep in mind that your seed only has to be buried no deeper that the width of the seed. So clover, rape, and turnips would all probablt work. But your safest bets are wheat, oates and rye. I understand that you don't really want the grains, but with no access to turn up the dirt, the grasses are your best option.