Nassarius snail and vaccuming while doing PWCs. You could also try a pair of two spot gobies, but I've never had longterm success with them. They sift sand. A bullet goby is much bigger, but I have no experience with them or know if they'll do in a 29g.
Those snails live under the sand and eat stuff so it might not help the top of the sand. Don't let anyone talk you into a sand sifting star either. It'll die real qwik IMO.
But I'd stay away from any cucumber.