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
Interesting fact that memory decay also is inherently logarithmic/exponential.
Learning logs with SRS is so meta.
BTW, your blog is one of my absolute favorites!
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.