A humble white t-shirt. Surely, the most versatile of items. You can dress it up or down, wear it with skirts, jeans, shorts or gym wear and it works well both in the summer as well as the winter. My favourite one is this loose, cotton tee from POP Cph. It has a tiniest bit of stretch and it falls beautifully. I wear it a lot, most recently with this powder pink skirt from ASOS.
T-shirt: POP Cph
Sandals: Sol Sana
And a few of the previous incarnations: