Shoot One and Only One Subject
Pick one subject, not allowing yourself to move more than a fixed distance (eg a 20-foot radius in a garden).
The subject should be a challenge—something ordinary, so that you’re forced to work hard to make an interesting image. Shoot it with as many lenses, angles, apertures, etc as you like, but restrict yourself to that subject only.
This exercise is a good one for those outings where you have other duties (eg supervising kids), where you’d otherwise be bored stiff. You can also do it at a restaurant table, lying on your back under a tree, waiting in line at a bus stop, etc.