Comments will appear when you hover.

Data Validation input messages will appear when you select a cell.

Gord Dibben  MS Excel MVP

Would a cell comment work? Right-click on a cell, then select Insert Comment
from the menu that appears. Enter the text you want for that cell, then click
on any other cell to finish creating the comment. When the cursor hovers over
that cell, the comment should appear. If it doesn't, select Tools >> Options.
On the View tab,
select Comment Indicator Only, then click OK. You can change the font, font
size, font color, transparency, etc. of the cell comments. You can even
insert pictures and graphs into them.

Hope this helps,

Hutch