Thermometers

a traditional minmax alcohol and mercury thermometer is cheap and quite light but fragile. of course it only tells you what the lowest temperature was

Take a look a coghlan's C-Tech Time & Temp Digital Dangler which less than $10. 24 hour max/min. Display. I don't have person experience with this item. I bring a bulker and more expensive portable weather station cause I also like to know other things.

–mark

A Taylor model 9878 *might* work, but I can't tell whether you can just leave it on and have it record the minimum temp.

http://www.taylorusa.com/foodsvc/thermo/digital.html

I have an old Casio nerd watch that will record temp (and altitude) in 30-minute intervals, so that's a possible avenue as well.

The taylor models should turn off automatically so you cannot get accurate min/max unless you stay up all night to click it on every few minutes.