you can breed your fish.. you pick a pair.. usually guppies work great. and you can show the life cycle. defects. and diseases that can affect the fish. you can show what the chemicals do to the water. so you can show the pH levels etc. oh and you can do a time laps of the female growing with her babies.. then when she gives birth then to the babies growing to adult hood. but then again it depends on how much time you have for this project.
or you can do something with plants. effects of chemicals or when CO2 is added to the water or you can do one with two of the same plant. in different tanks. one treated with the CO2 supplement and the other not. then with the other tanks you can also add the amount of light and how that affects the plant; the current etc.
A good standby has always been a clock made to run from the sugars in a potato. . Did they give any requirements apart from can't use animals? Does it need to be visual and have moving parts or is it more a photo journal? What is the desired outcome of the project? Ie to show you understand a particular scientific concept or just something that takes your fancy?
Or a really quick experiment to show how plants take up water/ nutrients is to have a white rose or carnation, slit the flower stalk in half (not all the way to the flower) and then put the two halves still attached to the flower into two seperate dyed water. Each half of the flower should become the colour of the dye used.
It's supposed to demonstrate the scientific method in some way. I've decided upon using chemicals to see which one would work best in an ice pack. I'm going to use sodium chloride (salt), calcium chloride (the stuff they put on icy and snowy roads to melt it), and ammonium chloride (haven't really researched this one much). Should be fun.