Hi
After she has her work permit, she can go to the U.S. anytime for a week or whatever. She'll need to show her valid work permit and passport to return to Canada.
After she starts working, you could get married in Canada. That's not a problem, but it makes things complicated because it will then appear to immigration that your wife intends to stay permanently with you.
Therefore, after you get married, you should immediately begin the sponsorship process. Your wife won't be able to leave Canada while the application is in process (well, she could, but she wouldn't be able to return until the application is finalized).
If she is working at a hospital, she will require an Immigration medical, therefor she will have to get her work permit at one of the US offices.
"Eligibility to apply as you enter Canada
You can apply for a work permit as you enter Canada, but only if you meet the following requirements:
* You are from the United States, Greenland or Saint-Pierre et Miquelon.
* You do not need a medical examination (or you already have a medical certificate).
* You do not need a temporary resident visa to visit Canada and your job does not need a labour market opinion from HRSDC. (See Jobs that require a work permit but no labour market opinion.)
* You hold a labour market opinion from HRSDC.
PMM