I just wanted to mention that this really depends on the zone you live in. Planting for the right zone will make plants easier to care for and require less watering which is good for the environment.

I'm in the midwest in zone 5 and these work well for me with little care:
Lilac
Roses
Sedum
phlox
tulips
black eyed susans

The phlox and sedum both spread rapidly for me. They make for pretty spring and fall interests, respectively.