I can strongly recommend memorising some logarithms for use in mental maths. It's given me powers I did not expect to have! Here's what I wrote about it when I started: https://entropicthoughts.com/learning-some-logarithms
It's been about as useful as one would expect. I don't need it daily, but when I need it, I can usually estimate a good enough answer in the time it takes someone else to do it on a calculator.
It has also helped a little with getting a geometric appreciation for numbers, but I suspect that could be improved significantly with more active practice.
