With flowers or fruit think of colours like a rainbow, red, orange, yellow, green, blue, indigo and violet. That would be the theme running through the pictures. You could use fruit like Apples, Pears, Plumbs, Peaches etcetc, as they all grow on trees therefore, trees could be the theme. Different fruits are used in drinks like wine, so wine could be the theme. Hope that helps and good luck.
As to choice of subject why not choose people? You could photograph people like a 20 year old, then 30, 40, 50, 60 and 70 years old with a theme of decades, that is ten years between each person or decades as born in, so a 25 year old would have been born in the 1980's whilst a 37 year old would have been born in the 1970's.