I finally traveled last weekend and went to Paris. For eight hours. Before my friend and I planned the trip, I didn't know how I felt about taking a less than one day trip, but it was probably the best decision we could have made. We tried planning our trip around this Huffington Post article, but it left a lot to be desired in terms of specific details and there were just some things we didn't have time for. So here's an incredibly detailed itinerary if you're planning on doing this kind of thing in Paris.
1. Take the first morning train out of your home city and arrive at Gare Du Nord at around 9:45. Immediately head to the metro and buy a single day metro ticket, which will cost you around 10 euros. You can go with the single day pass that covers Zones 1-3.
2. Take the 4 train towards Mairie de Montrouge for 3 stops to Strasbourg-Saint Denis, then switch to the 8 train towards Balard 9 stops to Ecole Militaire.
3. Get off at Ecole Militaire and, once you get out of the stop, head straight towards one of the many cafes that line the nearby streets to get some coffee and croissants.
4. Once you get your breakfast, walk towards the Eiffel Tower (you'll basically see it right after you get off the train). Hang out around there for a bit and leave at about 11:30. If you want, you can try to get in line to go up the Eiffel, but the line is long and you'll get a great view of Paris later on.