Set default value in cell?

There are two ways I can think of to do this.

Using VBA, you could run a macro when the sheet is opened to check if certain cells are blank - if blank enter the default value, if not blank ignore. The problem with this is that after it's been run once those cells are now not blank, so the next time the macro will ignore them, which probably isn't what you want. VBA gurus on here may have a better solution.

A better solution, I think, is to use a different cell for user input from the cell you use to record the input or default value.

A1: enter your default value, lock the cell
A2: leave blank, leave cell unlocked
A3: enter formula below and lock the cell

=IF(A2="",A1,A2)

In other cells where you need the result, refer to A3 instead of A2.
Protect the sheet so the users can only write into the cell(s) you want them to.