I know what you mean - that would make the spreadsheet interface clearer. I don't think it can be done (other than using a different tool like form tools mentioned... else some funky vba approach).
There are other visual cues you can use. I like to make my input cells green. You can also use cell outline and a prompt like "<=choose one" to make the interface clearer. Attached a very tiny example.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.