How can I use a Drop Down List and skip data validation in Google Sheets?

I want to show a Drop Down List in a cell in Google Sheets, where user can choose a list of words. But I want to let to user to input any number too. Since the only way I know to show off a drop down list is by setting a data validation, if user inputs something different from the given list it will display an error message (actually it should not be an error on how I pretend it to).
I know (and I have already done it) I can use a formula to achieve this behavior, but it does not show off the requiered Drop Down List.