I second the recommendation to stay overnight and head back to London the following day. Otherwise, you'll be checking your watch throughout the day to ensure you're on track & back in the queue for the Eurostar that evening. Staying overnight would give you the opportunity to experience one of the essentials - an evening Seine river cruise where you'll see all of the beautiful buildings lit up at night, some of the main sites, i.e. Notre Dame, and see the Eiffel Tower glimmering with lights & reflected across the water - all at a reasonable cost. This would allow you to have some time during the afternoon to walk into one of these museum dual ticket plans to avoid the long line at the Orsay and enjoy the gorgeous art in Paris. (1. 1st museum: Purchase your ticket at the non-d'Orsay listed. 2. 2nd museum: Go to the short line on the far-right at the d'Orsay with your ticket. Brief but wonderful view of some highlights: Go directly to the top floor & see the Impressionist art. Then take the escalators down to the main floor & see the beautiful sculptures. There's a sculpture straight ahead near the back that I have a difficult time believing it is made of marble! The draping of the shawl looks like gossamer fabric.
Musée d'Orsay - Musée Rodin Passport - 15E OR
Musée d'Orsay /Musée de l'Orangerie Passport - 16E If you do this one, sit down in each Monet oval for at least 10 minutes. Think about what's happening during that period of the day; what would you hear for sounds? smells?
After the museums, head to the Jardin des Tuileries park, across the river from the d'Orsay or go near Notre Dame or into the St. Germain neighborhood for some refreshments at an outdoor cafe. My favorite garden is the Luxembourg Garden but not sure that you have time to go there.
In your short time, try to sit at a cafe and enjoy being there vs. just checking off a list of items, so you can enjoy some of the atmosphere even for this short time.
If you can't stay overnight, you'll probably have to take one of the hop on/off buses to see highlights, instead.