Open telescopes
A couple of stripped down telescopes, just some lenses and mirrors, hanging in space, until you look through two of them and you have a large telescope.
Obviously it isn't a very practical telescope and you can't point it, but it is one that works and can show the difference between refracting and reflecting telescopes effectively, and can be cool if you hide something cool which you wouldn't normally find where it is pointing.
The objective lenses are just hanging so you can focus the telescope by moving them back and forth.
