I just put together a spreadsheet to demonstrate how some calculations work. Three of the cells act as inputs. The values in those cells kick off a cascade of calculations in other cells containing formulas. There are also a few cells with constant values (not formulas, but not meant to be modified).
In this case, because the spreadsheet is simple, I don't think people will have much trouble figuring out which cells they're meant to edit. But in general, what formatting should be applied to a cell so that it affords end-user input and experimentation?