This is a self-watering (aka bottom irrigation) planter that I created as an easy 3D print.
It can easily be scaled to fit your space and plant, although 50% is probably the realistic minimum size. The full size version works well on a window sill.
This is three-piece print is designed to print well on any 3d printer without supports. If you have it, it is a good idea to use a second color for the float to make it more visible.
More information can be found on the Easy 3D Print page. The STL files are available on Thingiverse.
The Circuit Playground board from Adafruit is a wonderful thing. It provides a self contained learning environment for Arduino with built-in environmental sensors, neopixels, and buttons and switches in a wearable format for $20. This is an excellent teaching tool that is cheap enough for students to take home with them and continue tinkering with. The board is event built into the current Arduino environment.
I have developed a introductory workshop to the Arduino world using the Circuit Playground. At the moment I am offering this workshop at the Columbus Idea Foundry. Materials for the course can be found under the workshop tab on this site.