RFM69 @ 433 MHz - basic packetized FSK/GFSK/MSK/GMSK/OOK radio at 433 MHz for use in Europe ITU 1 license-free ISM, or for amateur use with restrictions (check your local amateur regulations!)
The simplest RF modules are ASK modules. The receivers look like they are from the 80s. Many have the data pin marked ATAD because somebody got it wrong and others copied. They typically have a "superregenerative receiver", which is what your grandfather listened to before WWII. The problem is, you get the random frequency within a band that it generates on the transmitter side, and a receiver that receives anything in the whole band. If you need more than one radio link, skip ASK modules. One ASK module on eBay claims to have a superheterodyne receiver, which looks identical to all the other ASK superregenerative receivers and has nowhere near enough parts for a superheterodyne receiver.