I needed to display the temperature in different parts of the home office and ended up using a spare powerbank, Arduino, and DHT11 sensor to do it. I wanted to 3D print a permanent case for it but as I began designing it I decided to make it possible to include my phone and space for additional Arduino projects to be mounted to take advantage of the powerbank.
Eventually what emerged was a very large phone case with a rack system that allows you to load phones, powerbanks, and in the future Raspberry Pi projects (and more) in a very modular design. The phone can be charged by the powerbank, the powerbank can run Arduino projects, and the phone itself can also power very simple Arduino projects like the temperature sensor/display.
An older project, the Pro Micro-run mini-keyboard was incorporated into the case, meaning those keys you see actually work when you connect a mounted phone to the internal Arduino Pro Micro. It works the other way around too. You can connect the case buttons to any computer and use the shortcut keys (home/end/enter/etc.) or use that computer (if it has the Arduino IDE) to reprogram the keys.
A WiFi-enabled Arduino board could also connect sensor data to the phone or Raspberry Pi directly or to the cloud and then displayed on the phone (and anywhere else with access) sort of like we did with our weather station a while ago.