So a String is already a variable. You only need
str() if you are converting from other types (such as Numbers). So this is the same:
var thing = "Some String"
You can't add a String to a Panel. That doesn't make sense. String is just a string of characters (like a word or sentence) not visual Node. Perhaps you meant to create a Label. A Label is a visual Node that can hold text data (like a String). You can add a Label Node to the Panel. Then do:
var label = $Panel/Label
label.text = "Today"
label.visible = true
print("Today is visible")