Producing a simulation of a pond environment in a cup is easy and fun to look at. The method used at UW-Madison is called a hay infusion and despite the concoctions being made for over 50 years, something novel pops up frequently. The below directions describe how to make a hay infusion.
Go to your nearest body of water and collect a water sample. Any natural water will work, but tap water will not. Tap water is chlorinated to remove microbes and the water out of your tap will have enough chlorine in it to kill or inhibit the growth of microbes.
Pour the water into a glass, or disposable cup, and add a handful of hay or grass to the pond water. A glass you do not care about should be used, as it is going to get pretty scummy.
Let the mixture incubate at room temperature or above for several days. If desired, the addition of a light source (either the sun or a lamp) will encourage the growth of photosynthetic microbes.
During the incubation, check the infusion and add more pond water as it evaporates.
In 5 to 10 days the broth should turn dark and turbid. Examination under a microscope will reveal a large number of microorganisms.
Some water from Lake Mendota in Madison, Wisconsin was placed in a plastic cup with hay and incubated at 30°C for 3 days. Because of the carbon source, a huge collection of microbes grew and and the video shows all the little critters swimming around in that cup.