I have often wondered how cold it got inside and outside my tent during the night. My Casio Protrek watch has a thermometer. However, this has a couple of disadvantages. Firstly the sensor is on the side of the watch, so if you are wearing it on your wrist it just measures your external body temperature. Secondly, it only shows the current temperature and not the maximum or minimum.
If you have a greenhouse, you are probably familiar with the "U" shaped spirit thermometer, which has a metal cylinder on each column to indicate the min and max temperature. These aren't very practical to carry around in your rucksack.
Recently I saw a Digital Solar Thermometer on Amazon. This displays the current temperature and the minimum and maximum. It's small and only weighs 15g. You can choose to display the temperature in either Celsius or Fahrenheit and there is a reset button, which resets the min and max display. As far as I can tell, the min and max will be reset after 24hrs automatically. Checking the temperature against another digital thermometer it seems to be accurate.