A designer does not need to be a programmer but understanding programming is key. One of the most valuable but more expensive skills for a designer to acquire is programming...
Excel is the brick and mortar tool for the designer. Through really any decent spreadsheet software will do and have most the features I list, so pardon the Excel centric explanations. With Excel you can build entire complex systems and mimic game systems. Allowing you to not only prototype systems but also balance and analyze them.