Ah the ZX81 wasn't all that bad, very affordable when it came out (even cheaper if you built it yourself) and great for learning BASIC. But a ZX81 in 1990...

Actually neither was the Plus4, especially with a proper graphical operating system running on it in the form of GEOS (once the 3rd most popular OS in the world by sales).

Well basically you can fun with pretty much any old piece of junk if you're into that