crystal radio.It requires no external power, and you can build them out of junk parts like they did in World War II. That's right, GIs were building these on the front lines with stone tools and their bare hands, so you it should be well within your capabilities at this point.
This receiver has four components: an antenna, tuning coil, detector diode, and headphones. You also need a good ground connection.
The hardest to find item is a good set of headphones. They need to be high-impedance, at least 2000 ohms. Most consumer electronics headphones are 8-32 ohms and will not work at all. I use the single-ear headphone from a CDV-700 Geiger Counter and it works very well. Its impedance is about 4000 ohms. Cheapest route is to get a "crystal radio earpiece" from Jameco et al, but the sound quality sucks compared to a set of old-skool high-impedance 'phones from the early-mid 20th century.
The tuning coil is perhaps the easiest part to acquire. You wind it yourself on an oatmeal container or piece of PVC pipe that's a few inches in diameter.
Every few turns I pulled up a loop and stripped the wire for connecting an alligator clip.
The second hardest part of a crystal set is a good ground. I use an 8 foot ground rod hammered into the high desert sand outside my shack. It does an okay job, but I'll soon be supplementing it with some radials. Cold water pipes were often used, and that works well when you have copper plumbing that goes all the way to your well, and not plastic pipe.
The easiest diode detector for a beginner to work with is the classic 1N34A, aka NTE109. They work consistently well. I find a Galena detector to have a nicer, more "soft" sound, some enthusiasts like pyrite, but YMMV. Try a 1N34A first.
Finally, you have the antenna. Make it as long as you can, at least 100 feet if possible. It doesn't have to be up high. Rural listeners insulate the top strand of wire on their livestock fences and use that. A quarter mile of electric fence wire makes a really good crystal radio antenna.
Using my 18 oz Quaker Oats coil, a 1N34A diode detector, and 90 feet of wire with an average (not great) ground, I'm able to hear my local AM stations, and more than a few of the Class A blowtorches in the region. A longer wire antenna, better ground system, and adding a tuning capacitor would improve that. There is plenty of room for experimentation with crystal sets, which is something you should be doing.
http://www.hpfriedrichs.com/mybooks/votc/bks-votc.htm - The Voice Of the Crystal
https://www.midnightscience.net/ - X-Tal Set Society