The original microscope, as created by Anton van Leeuwenhoek (spellings of the last name vary) in the 17th century was nothing more than a drop of water (or glycerin, which doesn't evaporate as quickly) caught in a pinhole in a metal plate, with the sample mounted on an adjustable holder. The simple "lens" was held very close to the eye, and allowed examining an object at a close enough distance to see the "animalcules" (as he called them) in a drop of pond water.
A similar setup could be cobbled together using a drop of clear liquid, mounted very close to your phone camera's lens cover. The lens mount can be made from aluminum foil, held on the phone with tape. Careful cutting of the foil can avoid covering the camera's flash LED, allowing good illumination of the sample (possibly aided by putting frosted tape on the LED cover, to widen the illumination angle). Combine this with the phone's zoom, and you should be able to see individual cells in an onion slice or similar tiny details.