The Brits have a number of excellent turns of phrase, and one of my favorites is to say a person is "in good form," as in, "Giles was in good form at the party last night."
I like the implication that people comprise different forms-- the implication that one could just as easily be "in bad form," and that somehow that's natural and an OK part of being a person. That your friends can acknowledge when you're in good form, and in bad form, and that they understand you and accept you either way.
To be "in good form" sounds far more natural than its American counterpart, being "on your best behavior," which to me connotes more effort and strain than just happening to be in good form. You know?
Today was in good form.