While on vacation, I've never had a sim left behind change jobs or have an affair. By default, they care for themselves which includes attending school and/or work and taking care of needs. I believe there are limited actions you can control (such as having them build certain skills) which can be done by clicking the icon on their picture. Not positive about that capability, however, because I have always let them just take care of themselves, but it's worth a try.
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