Selected Answer
The attached workbook contains VBA code to make an ActiveX ListBox work almost like a drop-down, allowing multiple selections and displaying them in a cell. You may have to learn a little about the ListBox control.
To start you off, click Design Mode on the Ribbon's Developer tab. Then right-click the ListBox on the attached workbook and select Properties from the menu that opens. In the dialog box that appears you can adjust the size and location of the ListBox as well as its appearance. Observe that the ListFillRange property points to "List" which is the name I gave to the range A1:A10 on the worksheet. You might change the property's value to $A$1:$A$10 or move the range wherever you want in your workbook or assign another range altogether. You can modify the texts in the List range.
Read the remarks in the code. You will find the code in the code sheet of Sheet1. Note that it must be in the code sheet of the worksheet on which the ListBox is located. Also note that I assigned the name Selector to the ListBox.
Make sure that Design Mode is turned off. Now, when you click on the ListBox it expands and you can select or deselect items and they will be listed in cell B8. When you click elsewhere on the sheet the ListBox contracts to a single row.