I thoroughly hacked my C64. The main thing I did was add a static memory chip of 16k IIRC, half of which could be made to replace the portion of memory that stored the font, at the flick of a switch. The othre half could be similarly mapped on a different part of memory. A rechargeable battery kept the contents of that memory safe even when the computer was powered off. This was a godsend for game development: the ability to switch fonts without having to load them every time, and the ability to instantly get a bunch of utility and debugging routines without loading or even overwriting that part of memory sped things up quite a bit.
I do miss those days, literally spent in my parents basement, or at my friends place in the attic, listening to late night radio while hacking away.