The much maligned bussy lizzie is a versatile little plant for filling in awkward gaps in sun or shade. Avoid anything that says 'mixed' to avoid the OAP look (Sorry). New guinea impatiens are more attractive, expensive and slower growing.
The sweet potato. Available from most supermarkets. These will produce a climbing vine-like plant with attractive, small veined leaves. Tends to scramble so if you don't like this effect avoid this plant. It is a cheap experiment if nothing else.