Boot into a system which allows you immediate programming
Like Bash? For me, Linux is what made computing interesting and fun again. It has easy access to programming tools, and none of this forced separation of users and developers.
(preferably with a modern OO syntax) and access to video, sound and peripherals. If there's anything that has suffered over the past three decades, it's easy access to I/O.
I admit it gets a little complex here, but for example Python (a key element in my "fun computing" experience) has nice libraries for these. For example, some of my electronics/FPGA work owe a lot to Python's serial port module. Not because the serial port is hard to program otherwise, but for making it easy to write all kinds of code around it.
I have no experience in modern graphics programming. However, I have the feeling that the bar for awesome graphics is a tad higher today than it was in "the year 64". Today's awesome is rather nontrivial at the direct low level we associate with C64 programming, so even professionals use higher level tools. (I think my background in physics and math helps appreciate 3D graphics, for example coordinate transformations using matrices are a basic (pun inteded) skill but I imagine there are lots of programmers with no need to do it.)
Nevertheless, I understand the point about recreating an environment in the '64 spirit. There are several projects around, the two I can think of at the moment being http://sol.gfxile.net/gp/ and http://pelulamu.net/ibniz/ .