Lie your mic (without any stands etc) down on a picnic table, then drag something heavy along the surface of the table. I'd say a contact mic would work best with this, but anything would do really; I used the internal mics on my portable recorder.
I accidentally got some nice sounds this way, the vibrations in the table recorded by your mic will be a nice low frequency rumble. I personally was dragging a brick across a spare paving slab, on top of the table, with my DR100 on the table too.
EDIT: (The picnic table was also on paving slabs, but this may work with it on bare ground too)