Sometimes a "sketch" is a very involved drawing used as a preliminary study for a painting, but I like to think of it as a minimalist composition, just the essence of something.
I've started to "go sketching" instead of going painting, packing just a pencil & eraser instead of water, paint & brushes. I'm not sure it's always quicker or easier, but it does give you more of a feeling of freedom. You can erase, you can make it small & simple. You know it's not supposed to be a finished product. And if you happen to be in a crowded environment (or anyplace public) you can be unobtrusisve.
It's especially effective when doing animals, since they're liable to turn or walk away. You can capture the outline of their head or their back. Even flowers can move, too, if it's breezy or if they're the kind that folds up at night.
You can still get the same quality of focus as when you're painting, maybe even moreso, since there's not so much standing between you and your subject.