In MVC, how do I return a string result?

You can just use the ContentResult to return a plain string:

public ActionResult Temp() {
    return Content("Hi there!");
}

ContentResult by default returns a text/plain as its contentType. This is overloadable so you can also do:

return Content("<xml>This is poorly formatted xml.</xml>", "text/xml");

You can also just return string if you know that's the only thing the method will ever return. For example:

public string MyActionName() {
  return "Hi there!";
}

As of 2020, using ContentResult is still the right approach as proposed above, but the usage is as follows:

return new System.Web.Mvc.ContentResult
{
    Content = "Hi there! ☺",
    ContentType = "text/plain; charset=utf-8"
}

you can just return a string but some API's do not like it as the response type is not fitting the response,

[Produces("text/plain")]
public string Temp() {
    return Content("Hi there!");
}

this usually does the trick