Software defined radio is a lot of fun for me, there is interesting math, there are weird RF signals to investigate, and it is fun to decode things like ADSB flight data. That said, if you want to do real science, you can also do radio astronomy with these things. For a relatively small amount of money ($300 - $500 US) you can set up a radio telescope in your back yard that doesn't suffer from a lack of "dark sky". Also makes for a great science project!