Buy as many grades of sandpaper as you can get, preferably all the same type and manufacturer. These can be your stimuli. Make a testing box with 2 hand holes in it so your "subjects" can feel the sandpaper samples without seeing them.

Experiment: Is roughness related to grain size? Measure the average grain sizes with a microscope. Then let your subjects feel the different grades and score the roughness on a scale of 0 = perfectly smooth to 10 = roughest imaginable.
You should see that particle size correlates to roughness.

Experiment: Is roughness related to friction? Measure the amount of force it takes to drag a metal weight across the sandpaper. Do the rougher feeling papers have higher friction? You should find there is not much difference in frictional coefficient between the different papers.

Experiment: Can our ability to feel roughness be changed? Use 2 grades of paper that are very close together in roughness. Get your subjects to try feeling them (a) while wearing latex gloves (b) after soaking their hands in warm water (c) after resting their hand on a vibrating surface (causes adaptation).
Can they still tell the difference between the 2 sandpapers?