Again, sources from here:
1) Discussing this case of אות
The majority of Jewish commentators do understand that God made some sign on Cain's face or body, although there is significant disagreement in Rabbinic literature as to what this sign was (a horn, writing on his face, etc.), and they don't really explain why it would prevent him from being killed. This is what those translating it as sign would follow.
Kimhi, Ibn Ezra and others suggest that God convinced Cain that no one would smite him, using an אות to convince him (just as God uses other signs elsewhere to convince people). This is explained very nicely by MY Ashkenazi in his commentary, Hoil Moshe, where he suggests that the אות that convinced Cain was the previous comment, that a strong punishment would fall on whoever would attack/kill him, and that calmed Cain. According to this interpretation, treaty or covenant would work very well (in fact, Hoil Moshe groups this word together with Havtacha, or promise), although translating as a sign might still also work.
2) אות elsewhere in Scripture
Overall, when you think of this word throughout scripture, it seems to connote a reason to do an action, so it sort of serves as a reminder or reassurance, and is also usually something visible. The rainbow is an אות ברית, the sun and moon are אותות, etc. etc. (concordance here), and in all those cases it seems to be best translated as a sign (not as a covenant as you suggest from Genesis 9). However, in other places, (e.g. Exodus 31, where the Sabbath is called an אות), there seems to be no visible aspect of such a treaty or reminding/reassuring factor, it might be translated differently. However, even there it might be that seeing Jews keeping the Sabbath is a sign, so maybe not.
3) Other translations of אות
A quick web search shows "sign", "token", "guarantee", "miracle", "mark", an likely more.
(Note: The word אות is likely related to את, אותו, etc. and perhaps related to אוה, as noted by Brown, Driver and Briggs. )