What are some examples of minimum viable products that can be built in a day?

A power point presentation showing the problem that this product/service will solve

A video demonstrating the problem that this product/service will solve

A drawing on a napkin showing the problem that this product/service will solve

some clumsy screencasts for some famous products have been used to raise hundreds of millions. what counts is if you have a problem worth solving and if you can explain it in a way that people will understand.

Mind that there are too many factors to take into account that you need some professional point of view. I would definitely recommend to read about minimum viable product examples first. You see, it will save your time and money. Believe me you will find that information being very helpful and make your mind on the best choice. Hope it can help