Sketching things that keep moving is quite a good way to quickly fill a page or two in a sketchbook. In the past I’ve done this with people but recently on holiday in Cornwall I just looked out of the window of the place where we were staying and drew the seagulls in my tiny Leuchtturm sketchbook.
They were almost always flying past the windows or landing on lampposts and rooftops. Sometimes I might have only drawn a head or a wing before they’d moved or flown away again. Other times I’d merge a couple of birds together because it was impossible to draw from just one as they swooped by.
It’s definitely a good way of loosening up my drawing too.