A pencil by the window
Place a pencil upright by a sunny window. Mark the tip of its shadow on a sheet of paper, then mark it again an hour later. Write the times beside the two marks. A small change in the room now has a record.
Keeping a field note
Start with one detail that would be hard to remember tomorrow: which side of the stone was dry, how far the sound carried, where the light stopped. A single sentence is enough.