Have a look at the sporophyll of Bracken under the microscope.
Have a look at Cardiac muscle under the microscope :).
Caffeine, the popular additive in all sorts of beverages (including many of my favorites) looks stunning by polarized light microscopy!
Benzoic Acid is a simple aromatic carboxylic acid, with a crystalline structure that lights up in wonderful colors.
Sodium thiosulfate is a inorganic, white crystalline substance that displays incredible birefringence, why not have a peek?
Ever wondered how I do my focus stacked microscope images, have a peek at a quick write-up on the procedure I use.