Each time I am out hiking, generally testing gear, the few things that I miss are the ability to accurately record the temperature and barometric pressure as to get an idea of how the weather is changing.
Temperature min and max is useful to record in trail journals (what ever form that takes) so that trip planning can be made with more knowledge. Each year I try to take in a weeks walk with fellow hiking buddy Andrew and it this does help with the planning. Also good for some overnight trips so that gear lists can be revised to include suitable clothing, bedding and rain protection if needed.
You could use the traditional mercury thermometer but they won't give a min and max, just what the current temp is, which is not much good during the middle of the night or early mornings which are generally the coldest.
So, this week I purchased a Suunto Core Lite Black, second hand from the BPL forums, cheaper by far that anything I could get locally here in Australia with all the features that I want.
I will post a review of the watch when I get a few km's out with it.