I've glued about 10 toadstools so far with cyanoacrylate super glue to rock before with no problems. They won't be terribly happy for a day or two, but they recover quickly.
You can also take a plastic toothpick, shove it through the base, and glue the toothpick to a rock. The toadstool will grow over the toothpick and attach to the rock naturally.
You can take those plastic paper binders (the ones that slide over a stack of papers on the side...like a term paper binder), cut a small piece off, and clip it over the toadstool...and glue that to a rock.
You can also take two pieces of rock, squish the toadstool between them (but not too tightly), and rubberband the rocks together. Eventually the toadstool will attach to both rocks & you can remove the rubberband.
Hope that helps!