I always thought it would be fun to create a vacuum tube computer in which the primary computation is performed by a couple of vacuum tubes (i.e. forming a single NAND gate). All of the intermediate results, would be handled by a regular microprocessor but the vacuum tubes are employed to perform the computation. It would glow and look cool (maybe).
trouble with tubes is they look the same whether theyre idle or busy. I like relay compututers, since the clacking really is the computation, they're a step above LED indicating the state of transistors, since compute would happen with or without their indicating.
Also, maybe: build some op-amp analog adders, multipliers, and wire them into a digital computer via ADCs and DACs for your math functions. (Could also do log, antilog, maybe other functions as well.)
