Before applying the data validation, you can enter a formula into the cells to show the default value when the row is active. Let's say the key column is A, when you start filling out data in column A, this default value should appear.

First, enter a formula like:
=IF(A2="", "", "not done")

Copy that down your column.
Now apply the data validation to the same cells.

Anytime you start a row, "not done" will appear, but you can use the DV drop box to change the value. When you do, the formula that was there is now gone for good.