My fiancee is here in the US from Canada on a K-1 visa. We haven't married or applied for the AOS. Can she travel to Canada and re-enter the US without additional paperwork (or problems)? If so, does it matter if she drives or flies?

When her visa was originally processed at the POE the immigration officer mentioned something about it, but we were to excited to be approved to remember anything specific.

K-1 is a ONE time use visa, if the officer at POE took the immigrations packet, the visa has been used, and if you leave the country before marriage and before the I-94 expires, the consulate "MAY" reissue another K-1 that expires on the same date as your current I-94 card. If not you will be starting the K-1 visa process all over again.

Like Dan said, no guarantees that the consulate will revalidate the visa for a second entry and the ORIGINAL 90 day clock remains intact, as the second entry (if allowed), does not begin a new 90 day marriage clock...

And at the interview, when the visa was approved am certain the officer 'mentioned' something about it then too

As well, if she just wants to go home and visit immigration is unlikely to be sympathetic and may well refuse to re-validate the visa. If it is a family emergency - death of a family member, etc. - then they are more likely to be sympathetic to her situation and re-validate it. The reason she 'needs' to return to Canada will be important in their decision. Since there is a 6 month 'window' during which to activate the visa, Immigration expects people to be able to take care of any necessary business prior to their entry to the US so that they don't need to leave the country after their entry.

The K-1 Visa is good for a one time entry only for the purposes of getting married within 90 days, and filing for adjustment of status to become a permanent resident of the US. If she wishes to return to Canada for a visit, your best option is to get married, file for AOS and include an application for the I-131- Advance Parole.