This question is basically covering what are the best companion plants to grow with lavender. These are plants that like similar growing conditions, and look good next to one another!
Rosemary and sage both have similar growing and maintenance requirements, as they all come from Mediterranean environments. They all tolerate dry soil well, which means they grow well in the same space.
As well as this, they all have wonderful and enjoyable scents, which produces a beautiful perfumed area.
Alliums are also drought resistant, and help to deter pests from the garden, which makes them another great companion plant. These can be grown underneath the lavender so the blooms grow through the lavender foliage.
Sedum also like low-fertility, alkaline soils, but also enjoy drier climates.
The traditional olive tree is another great option, also from the Mediterranean like the lavender. The lavender attracts bees, which also does wonders for your olives.