Fun recollections. I remember my share of typing in hex codes in a C64 from magazines like Computer & Video Games. 
I recently stumbled across a quirky Youtube video which piqued my interest. In the video James O’Grady demonstrated a 3D maze game. He’d typed in the code for the game from a familiar-sounding book called Graphic Adventures for the Spectrum 48K.
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James must’ve been incredibly patient to type in all the raw numbers for the machine code with only very rudimentary checksums. And, given that the game is written entirely in machine code and the assembler source code is lost in the mists of time, I was impressed that James managed to successfully modify the game in a couple of different ways using a load of judiciousPEEKs andPOKEs and apparently without the use of a disassembler. In particular he’s written a nice maze editor program which runs on the Spectrum and allows you to design your own maze. I was quite amused to learn that the maze had to be square - I can’t imagine it would’ve been much harder for me to have allowed rectangular ones!