Anything that can measure angles and lines will do for me
For angles, it's useful to have an 180-degree offset scale on the instrument as well, where 180 degrees is due North. For lines, a ruler that measures in nm, on the scale of your chart (usually 1:500 000) is easiest. But even a piece of string works, since the scale is on the map itself, and you can also use the side of the map (degrees and minutes of latitude) to measure the distance.
Speed/time conversion scales on the ruler themselves are pretty useless, since ETE is influenced by wind. So a straight time/distance conversion scale on the ruler is only useful in nil wind conditions. Any time there is wind (basically all the time) you need to work out your GS first, which requires a flight computer of some sort (analog or digital), and who will do the ETE calculation anyway.
For diversions, I'm typically too busy to look start breaking out toys like rulers and protractors. The upper part of my thumb is 10 nm/5 min, I guesstimate the direction and for the rest I'll either use GPS, radar vectors or "I follow roads, rivers and railways".