Staz-on Ink
Acrylic Paint
Permanent/Alcohol Markers
Those are 3 things I believe work on non-porous surfaces, such as acrylic.
In some cases, you might have to do a test to see how well they "take" to the surface; and in some/possibly all cases, no matter what you use, friction or aggressive rubbing might remove it.
You could also Xyron or Modge Podge patterned papers to the surface so they show thru to the other side.
Have fun.