Some ideas from the been-there-done-that experience collection:
Thread guages are worth having, even if you can tell most of the common sizes from memory and experience.
An array of magnifiers are also useful tools, especially "optivisors" you wear on your head. I like the ones that have interchageable lenses for different powers. Also, if you wear eyeglasses, spend some time explaining what you do and what you want to see to your optometrist or ophthalmologist when you are examined so a prescription can be tailored to your needs rather than to what most people get: This may mean more than one pair of glasses, but well worth it.
Look at spiral flute taps and forming taps, as well as other special-purpose cutters.