Any ideas for an interesting project I can take up
Completely depends on what you find interesting.
Your own build led-cube is on many peoples lists. After building it you can use it as generic output device to show different patterns when different things happen.
e.g. red flashes when mail arrives, green lines when the lawn needs to be mowed etc
A morse decoder is also great and far more difficult than a morse encoder. You can use it to open doors or control an MP3 shield.
You can use an Arduino to determine the point of impact of shooting games.
You can connect your coffee machine (whatever thing) to the internet and smartphone.
A weather prediction station (fits in an UNO)
A satelite tracking system
A days of your life counter
A mood device, that can show your mood, pulsating on your heart rate etc.