There are a number of discussions on how to add tooltips to ALV grids. They all end up in one of two solutions:
1. Tooltips for column headers - these happen more or less automagically, but you can apply tweaks using field TIPDDICTXT (related fields are COLDDICTXT and SELDDICTXT).
2. Tooltips for cells containing icons - the icon is displayed with no text, and a tooltip is shown when the user floats the mouse-pointer over the icon.
In every discussion I have read today, I see the question asked "I don't want an icon, I want a customer number in the cell, and the customer name in the tooltip" - or something of that nature.
To the best of my knowledge, the ALV grid does not provide for the requested option. However, there seems to be a bug in the icon/tooltip implementation, which allows you to fudge it a little.
If the ALV-cell contains the text below, then the cell will contain a red-and-white icon (icon 39), then the text "100216", and finally the tooltip "B.P. Jones-100216".
@39\QB.P. Jones@100216
It seems that the second @-sign triggers the bug, so that any following text is shown after the icon, as well as in the tooltip. Maybe this is not even a bug, just an undocumented feature.
Finally, if you don't want to see an icon, you could use icon 5F (aka ICON_SPACE) - it's not perfect, just invisible.