Have a peek at Potassium Carbonate; a common buffer, drying agent and fire extinguishing medium under the microscope.
Stearic acid, a common surfactant obtained by saponification of animal fats and oils lights up under polarized light.
Nicatonic Acid, or Vitamin B3 looks interesting under the microscope and displays some birefringence.
Salicylic acid, a key ingredient in anti-acne products looks quite interesting under the microscope, displaying a small amount of birefringence.
Have a look at the sporophyll of Bracken under the microscope.
Have a look at Cardiac muscle under the microscope :).