Students use simple radio communications equipment to send messages to and from the ISS. For the radio, they use software running on a computer, in a system called software defined radio (SDR). They also use a small hand-held antenna, that they point at the ISS when it flies over. They learn about radios, communication, software, ISS, orbital tracks and related STEM fields. They need a ham radio license (or an affliation with a local ham) to send messages, but can receive with no license. Cost: $$
Also from ARISS: students talk by ham radio with astronauts on the ISS.