A good, respondent-friendly way of ending a survey is with an open-ended question asking respondents if there is any further feedback they'd like to make about the topic or organisation that has been the subject of the survey.
This type of question allows respondents to give more in-depth responses than the questionnaire allowed them to or to tell you more about subjects that weren't covered by the survey. They can also reduce any frustration they might be feeling that the questionnaire hasn't quite covered the topics they’d like to talk about. Remember that what’s a priority for you may not be important for respondents. The responses can also be a rich source of insight covering areas that you might not even have thought about.
Below is an example of an open-ended questionnaire used to end a survey: