you could use different fonts in the cell
and then choose the tick and cross - characters
for example
Marlett font
use an "a" to get a tick use a "r" to get a X without the quotes
then you can use
a
r
in a list and use that for your data validation
there are lots of other fonts that will provide a tick and a cross
OR
you could use conditional formatting and the ICON set to get a cross or tick that way
along with a data validation list
and use the icon set to NOT display the cell value - just the icon
may look better