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Another neat old-school calculator is the mechanical Curta: https://en.wikipedia.org/wiki/Curta

The Curta is the most famous, but there is a whole range of different designs: http://www.vintagecalculators.com/html/mechanical_calculator...

Most of them are pretty cheap on Ebay, <$200 not including shipping. The Addiators in particular are simple, cheap and small.