I am attempting to create a dropdown box in a single cell that has check
marks, for the following use:

I need to know what days of the week people are available (e.g., 1 = Sunday,
2= Monday, 3 = Tuesday, and so forth). In the drop down I would like to have
the drop down allow the user to click on the days they are available (e.g, 2,
3, 5 or Monday, Tuesday, and Thursday) and have the result in the cell be 2,
3, 4 (or Monday, Tuesday, Thursday).

Is there any way to create a drop down with checkmarks that returns a result
in the cell to this effect?