The SDR is fun on its own (I've browsed the local airports' signals and decoded APRS signals with DireWolf, using a nothing-special TV-style dipole antenna that comes in the box, and you can always use it as a decent FM reciever on your PC), but an interesting step up might be to also promise a basic handheld transciever (like $30) if the kid wants to get on the air rather than just flipping channels. (Studying for and getting a license is an activity in and of itself)