Daylillies are great plants for full sun. They increase, come back every year and come in lots of different colors,sizes and forms.
Hostas are nice for semi-sun to shaded areas. They also increase, come back and have many colors too.
Zinnias are easy seeds to plant. They come in lots of colors and sizes. Cheap too.
Sedums are great groundcovers that tolerate sun and no care. Sedums are also available in many colors and types.
Remember experiment, you can always dig things back up and change what you don't like.