You could mix a little pumice sand into the coffee soap and then you've made your own version of "Lava".
The coffee grounds provide nice abrasion.
I used this recipe:
125 g canola oil
125 g coconut oil
125 g Crisco oil
125 g olive oil
90 g NaOH
190 mL triple strength coffee
1 tbs fresh coffee grounds
Note: let the coffee cool before adding the NaOH. If you do not, the heat generated by adding the NaOH will cause the coffee to boil violently and spatter.
Crisco has hydrogenated soybean oil in it. By bubbling hydrogen through the liquid soy oil, it becomes solid. That helps make the soap more solid.
500 gram batches like this one fit nicely in a mold made of a 9" x 3" x 2" Rubbermaid drawer organizer plastic tray, lined with Reynolds freezer paper.