I definitely spent a full day at the Louvre and then bits and pieces of time that I strung together, on different days, to walk around the gardens above ground.
Just outside of the Pyramide du Louvre is the L'Arc Du Triomphe de carrousel (a mini L'Arc De Triomphe!).
I highly suggest buying tickets online before you go anywhere if you can. It usually saves so much time, as there is a line-up for people who already have tickets and another for those who have yet to buy.