In this project I take an old Motorola Bravo pager and replace its insides with my own PCB that fuses an ATmega328 (with Arduino bootloader), Bluetooth Low Energy, a vibe motor, a piezo buzzer, and some ws2812b leds. The original form is maintained, including AA battery power and two external buttons.
In it's current state, the pager beeps, blinks, and buzzes whenever I get a text message. It's a bit silly, sure, but a fun exercise in reverse engineering and design.
This is primarily a hardware project, so minimal effort was put into the Android and Arduino coding.
@josephchrzempiec I wrote my own android application to interface with the bluetooth module. It was very ugly. I'd check out one of Adafruit's examples. This might get you started: https://github.com/adafruit/Bluefruit_LE_Connect_Android
For the arduino sketch, Adafruit has a nice tutorial here: https://learn.adafruit.com/getting-started-with-the-nrf8001-bluefruit-le-bre