What types of vehicles can potentially be used in public places without any licenses?

I ask this because I think that machines that assist with walking are not far off. It is not to absurd to think that these ambulatory augmentations will create some safety concerns. That’s where licenses come in.

What are the parameters for vehicle use at our own discretion without need for permission or licensing by the government?

For instance stilts, bicycles, tricycles, unicycles, etc. Maybe something that bounces, a pogo stick….an inflated rubber ball that can be driven…a skateboard…roller skates…

What if one of these simple types of transportation added sufficient speed to make them dangerous? Maybe the razor scooter craze is a good example. Aren’t there motorized razor scooters? Are those something you need a license for?

Where and what are the rules governing what types of transportation is regulated?

I know that you can fly things under a certain altitude without a license for starters.