• Note that you can actually Fill a path that isn't closed... the results aren't necessarily what you want, though.
• So, to just make a line, Fill with None, and Stroke with a gray scale (or 100% black) and a particular line weight.
• Keep in mind that AI uses the last Paint dialog box setting you made to fill/stroke each subsequent path you draw.
• To change multiple paths' stroke and fill pattern, option-shift-click all paths you want to change, then set Paint to your desired filling/stroking options.