Lavenders and olive trees are a classic combination. In Europe they plant lavenders around the olive trees to attract pollinators.
Another idea would be grey Santolina 'nana', the low kind. The grey green of the olive leaves and the grey Santolina looks very well together.
Germander is a nice groundcover for dry areas. Bronze Carex such as Toffee Twist, or the dwarf red Pennisetum, 'Little Red Riding Hood'. Or a low boxwood hedge as a frame around each tree. A dwarf Phormium such as 'Tom Thumb'. The variegated true myrtle, Myrtus communis 'Variegata', or Myrtus communis compacta 'Variegata', clipped into balls or domes. Lonicera nitida 'Lemon Beauty'. A sedum, such as Sedum rupestre 'Angelina'.
Color? Geranium 'Rozanne'. A mass planting of Daylilys, such as 'Butterscotch Ruffles', which blooms year round here.
So many plants, so little time...
Q