I built a small portable temperature and humidity sensor using an Adafruit Feather M0 Express, a FeatherWing 128x32 OLED. an SHT31-D Temperature & Humidity Sensor Breakout, a JST 2-pin Extension Cable with On/Off Switch, and a Lithium Ion Polymer Battery - 3.7v 500mAh. I am going to add a FeatherWing - RTC + SD Add-on, so the sensor can time stamp and log temperature readings. I also see the possibility of adding an Ultimate GPS FeatherWing so the location of the reading can also be logged.[
I want to eventually build this into a small case to make it easier to take with me. The OLED FeatherWing also has three buttons that can be used to control various functions. I would use Button A to show the current reading on the display so the display is only on when it is needed. This would save on battery usage.
I have this running all the time in my room so I can always know how hot and humid it is here.