Here are a couple of smaller paintings I did today, using only painting knives. Sometimes, I like to approach things with a looser style and forgo some of the finer details.
The knives always yield a more impressionistic result.
I also tend to use brighter colors when I paint this way, since I'm trying to achieve something not quite so "literal". It's fun to push the envelope a little.
If you're in a painting rut, or itching to do something different, give this knife approach a try.
Just have fun with it and see what develops!