I have some more complicated conversions in my head but, for a lot of purposes, a liter is about a quart, a kilometer is about half a mile, and a kilogram is about two pounds. Those suffice for a lot of rough conversions I encounter when traveling. I suppose I could sit down and just memorize some F to C conversions.
Miles and kilometers can be estimated by the Fibonacci Sequence. The conversion (mi -> km) is very nearly the golden ratio (1.61 and approximately 1.62, respectively, IIRC). For any number in the sequence taken as miles, the subsequent number is the distance in kilometers. Your way is probably quicker, but it’s a fun bit of information.
Room temperature is 21°C and every degree Celsius is worth about 2°F; that’s about all it takes.