Two big ones. I bought the VIC-20 shortly after introduction when I was 21.

Big memory 1: writing machine language programs without the aid of an assembler. I couldn’t afford the assembler cartridge, but I wanted to break out of the BASIC sandbox.

Big memory 2: finding a military surplus acoustic coupler modem and using the schematics to make my own connector, then writing a terminal program so I could dial in to these crazy things called BBSs.