Preview feature to view intermediate values

Would be great if you could preview a block, meaning: show a depiction of its value.

The depiction would depend on the type, and its dependence on any local variables. If it's a picture, color, number, or text, just show it. If it's a list or tuple, provide a UI for flipping through the elements. If it's a function, or depends on local variables, then things get harder - we'd need to provide a way to give values for these local variables. I imagine it would be okay if, in many cases, we just gave up and said "No preview available for this type."