Below the top structure I mounted a long power outlet bar to have a convenient power location for whatever corded tool I am using at the table. Below that is the lower shelf which I built specifically to house my X-Carve CNC machine. This machine has 1000mm rails so making a dedicated cart for it would take up quite a bit of valuable floor space. The idea behind storing it on this shelf was so that I could easily slide it off and place it on the top work surface to use it. However, I haven’t actually removed it from it’s current location yet. I can use it where it is with no problems.
You can also use Google image search to research a project. Unlike the PDF search, the image search provides a photo of what the project will look like, which is helpful in determining if you want to pursue building it. By doing an image search for how to build a step stool, you will have a ton of options that all lead to woodworking plans of various quality. (The how to part of these is important—otherwise you’ll just get pictures of step stools.)