What's the easiest way to copy the current URL in Fluid?
Developer of Fluid here.

Yes, there's a very easy way: 

⌘ + L (That's "Command - L")

That will temporarily "show" the toolbar (if it was hidden), and select all the text in the URL location field. From there you can copy it, via ⌘ + C.

Then, to hide the toolbar, you can press:

⎋ esc (That's "Escape")

If the toolbar was previously hidden, this will hide the toolbar.