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A little more than 50 years ago, Intel released the 4004, a milestone in microcomputing history. The data bus had only a width of 4 bit. The address bus could access 2^12 addresses. The clock rate was 3/4 MHz and still you could get away with it and beyond.

If you still like to work with modern incarnations of the Z80 or the MOS6502, you will have lots fun with the 4004 as the quasi-grand-grand-father of the x86 architecture. If you still have one in your tinkering box, can spare a few unused breadboards, and feel like experimenting, the following might be a good place to start.

Mark Ablovatskii spurs the good thing - in connection with a little periphery and with the help of sensibly strung together assembly instructions - to calculate a limited but not unimpressive series of decimal places of Pi.

Exciting experiment with a 4-bit CPU. The details can be found in Mark's recent and apparently first post.

Great start, hope to see more.