You can turn on change tracking - an option under Tools in the menu.
You can lock cells using Format | Cells and going to the [Protection] tab.
Actually all cells on a sheet are locked by format, so what you really have
to do is select the ones you want to be unlocked (so that user or code can
alter them), then go there and clear the Locked box. THEN you have to apply
worksheet protection, with or without a password, to make the Locked state
effective.
Keep in mind that this is very weak security - passwords for worksheets are
easily discovered with a large number of tools readily available on the
internet.