The simplest project in Arduino that one could learn is to control the on-board LED via Blynk app.
It is a lamp or desk lamp that is controlled by your phone for when you need the light to get to your bed, but not once you are in bed.
A smart Christmas tree controlled by your smartphone using Blynk and ESP8266.
I made LED EyeBall controlled by Blynk using Adafruit Feather 32u4 Bluefruit LE. I can easily using Blynk!!
I changed ordinary boring desk lamp into Blynk controlled device :)