A bit of weeding, except I noticed that the 'weeds' in a courtyard area at the music school were self-seeded Verbena bonariensis. So I teased them out from the cracks as carefully as I could to preserve the roots where possible. This yielded twenty or so young plants which I potted up - about a hundred pounds' worth at current prices- reusing old pots, compost made on site and rainwater from the water butt. Hyperlocal and sustainable!