Well, in general, Sandbox - Shared Users provides the most isolation. But remember that things in your Home folder are going to get changed. So if there's data stored there (etc), and that shouldn't be touched, it's not something that you should do.
But what you can do is share your users and then create a guest user, and install/use that user... but you always have to boot into the Sandbox to be isolated.
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--Dave Nanian
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