Arduino Uno - What can I make with just the base board
Well,

Your question is very broad. But the Arduino essentially needs controlling gadgteers to make things easy ( and funny ).

However if you know a bit of electronics, then you may be able to use the IN/OUT analog ports and build other interfaces to control, for example the lights of your home, or even better. You can measure and control priate things should be linked. Increasing the power of your home.

If you have interest in cars, then you can perform to connect the on-board computer of your car to the Arduino, like this.

Well, there are numerous ideas, none of these I'm saying is new and there are even libraries ready to be used in these segments mentioned above.

Regards,
Andrew Paes