Choose something that you're familiar with and it would be fun for you. Take something you have and design a better version of it.

Do you play computer games? Design a better controller; different size/shape that fits better in your hands, move the buttons to better positions, change size to make them more comfortable to reach.

Obviously you use a computer. Design a better keyboard. Or design a new, light-weight monitor. How about a new mouse that fits your hand like a glove.

How about cars or motorcycles? design a better wheel or a new tire pattern, or how about a steering wheel that isn't a circle? Maybe a better dashboard console with everything in a better, easy to see, easy to reach location.

It takes practice to develop a skill. Don't just copy something that already exists, but try to design something different.