This may not be the most original idea, but it just occured to me anyway
You can get cheap ($22) Software Defined Radio (SDR) USB dongles to receive and demodulate almost any radio transmission. Basically they are digital TV receivers that can receive quite a wide range of the RF spectrum.
Couple one of these with a Raspberry Pi, screen, battery, speaker, and slap it in a case, and you'd have a really powerful radio scanner or spectrum analyzer for about $150.
A few links
https://learn.adafruit.com/freq-show-raspberry-pi-rtl-sdr-scanner/overview
Portable RTL-SDR Running on Raspberry Pi - rtl-sdr.com
RTL-SDR used as a Cheap Real Time Spectrum Analyzer - rtl-sdr.com