From the garden: Very thin cross sections of herbaceous plant stems and roots.
Try to slice the epithelium layer on the ventral(bottom) surface of a leaf and see if you can identify the stomata.

From the human body: Cells can be easily removed from the inside of ones cheek by gently swabbing with a cotton "qtip" (earbud). Also, blood, and hair (try and pull out the follicle too (ouch!)) make interesting microscopic targets.

From the lake: Fish scales have concentric ridges one can count under low magnification to determine the age of the fish, much like the rings in a tree. Fillamentous and free living algae

The possibilities are really endless.

Mintaka