Keyboard shortcut to select all text in a cell in Excel
The quickest way I've found is the following:
1) tab/select the cell, 
2) hit backspace to delete the contents of the cell
3) type Cntrl+Z to undo delete

When the undo function puts the text back into the cell, all text within that cell is automatically highlighted.  Then you can just type Cntrl+C to copy text only.  

I found that I needed to highlight all text in a cell in order to be able to paste it back inside another excel sheet whose cells already had other text that I had to leave intact.  

Hope this helps!