Radio transmitting. Its a quite large rabbit hole. Right now i upload some data of ships i receive with a small setup and earn a test crypto currency.
But you can do loads and loads of things, rtl-sdr.com is a nice cheap start.
Oh and locally running AI models. (Via GPT4All) Insane how far we are.
Like i said, look into GPT4All. At least for text gen. Open source and really simple. Download, install, choose a model (for mediocre laptops 4-8B ones are a good size) and chat.
For radios, there are some cheap beginner devices as starting point. Look for the rtl-sdr blog v4 usb dongle for about 40€. You can receive ~1-1700mHz with it, which are loads of interesting frequencies. You can buy a 30€ quansheng uv k5 or uv k6 (which is the same model somehow) with custom rom for a great handheld vhf/uhf radio. You can buy a 30€ SI4732 based ATS-Mini as "world receiver" also with custom roms like hjberndt's.
You could invest some more and get a flipper zero for around 250€ which is a neat tamagochi like IT-Swiss-Army-Knife.
Or a portapack H4M (the new cliffort heath version for the best hackrf one clone) for about 200€. Its the "big brother" of the flipper zero with fascinating capabilities like scanning surrounding ships and planes.
Dont forget proper antennas which can escalate quickly.