A thick layer of grass clippings works well. It will leach less from
the soil if you pile them up somewhere else and let them dry out a bit.
I then take a pitchfork and pile them on as mulch about six inches
thick, sometimes thicker, depending on the height of the veggies i have
growing there. Of course you wouldn't want to use grass clippings if
they've been treated with weed killer or other such things. Otherwise,
they work fine as mulch. I actually find them easier to work with
than chopped leaves, which mostly go into the compost pile instead.