The Library of Congress has a great page on the Smallest Flowering Plant in the World! 

Duckweeds in general are very easy to grow, inexpensive to attain and fun and interactive for students to handle. You can do experiments with duckweed species in paper cups on a sunny windowsill. Experiments can be simple; like putting duckweed into different sports drinks, sodas, etc in order to see if (or how long) the duckweed can live; or like growing duckweed in different light conditions (see picture). Experiments can also be very complex; identifying other plants which might be producing compounds to inhibit competing plants, then using organic chemistry techniques to separate different fractions of compounds, and testing those compound fractions on duckweed to see if growth is slowed or halted. 

This Duckweed Enthusiast has compiled a list of different activities, labs and experiments! There's something for everyone! 

There's also a Wisconsin company using duckweed to produce enzymes. This is a great example of how a pest plant can be used to help save the environment thanks to biotechnology.