Sunnah generally refers to something that was either a regular practice of the Prophet (that hasn't been otherwise qualified as something exclusive to the Prophet), or something that he encouraged his followers to do. For things that he did that weren't a regular practice — either things that he is only known to have done occasionally or even just once, or things he did one way some times and a different way other times — we can generally understand that such actions are not necessarily encouraged as sunnah, but they are permissible.
As for things that are a regular practice, whether or not it's considered "sunnah" would also depend on whether or not it was just a personal preference (e.g. his favorite color is reported to be green, but wearing green clothes is not considered sunnah) or simply a common cultural practice (e.g. riding camels was simply the common mode of transportation during the Prophet's time and not considered part of the sunnah); again, such practices would still be considered permissible, but not necessarily encouraged.
Exactly where this line is can be a bit harder to define and requires knowledge of the actual culture and behaviour of the time as well as the personal preferences of the Prophet himself; for this reason taking any single hadith as proof of "sunnah" — no matter how authentic it is — without understanding the proper context is often a bad idea.