To be honest, this tip is probably more for me so that I remember and don’t forget how it works. 🙂 There are times when I want to have multiple lines in a spreadsheet cell, and not just lines that wrap around. To get that hard linefeed, you just have to use ALT-Enter, and you’ll have a new line in your cell. Here’s what that looks like…
In my example here, I have one line of text in my cell, but I want a second line. If I press the TAB key, I go to the next column, and the Enter key takes me to the next row. But here’s what happens when I use ALT-Enter:

I now have a second line that I can type into, and it shows up as I want:
