Some inexpensive digital P&S accessory lenses (the ones you attach to the front of a Coolpix-type camera) actually fit the Holga--you can literally tape them to the front of the Holga's lens barrel. I've used a wide-angle one on one of my Holgas and it sharpens everything up quite well. Of course, I've had to adjust my visualization because the lens produces a wider image that what's seen through the Holga's viewfinder, but that's not all that terribly difficult to do.
Of course, there are those who say that sharpening the image like this is very un-Holga, but that's another argument for another day...