Little things
I have some backyard mud sitting in water I got from the tap a few days ago. Every day I inspect a drop of it under the microscope, hoping to see some protozoa swimming around. (A commenter identified my favorite, the shoe-shaped one, as Paramecium). So far though, all I see in the artificial 'pond' are single-celled plants (algae?) and some cool mutli-cell plants (picture a string of cells in a straight line). But no swimmers. I did some research on the web and found recipes for making paramecia. Basically you put some food (oats, or dog food or whatever), in water for 7 to 12 days and let the bacteria bloom (the water clouds up). Then when it starts to clear that's a sign that the paramecia are chowing down and replicating geometrically.

I would also like to grow some amoeba. But I am not sure how to do that. Probably the same deal.