Paris -- it has been said* -- is always a good idea. Even when you only have one day to bask in its charms. 
Carrie Seim on her one-day jaunt to Paris.
This summer I hopped aboard an early morning Eurostar train from London, with only my handbag and the joie de vive necessary for a single, delicious day in Paris. I was back to London by 11:00 p.m. It was exhausting -- and trés magnifique.
More surprisingly, it was also a bargain. By skipping a hotel stay, zipping around on the metro and indulging in how-can-something-this-good-be-so-cheap pain au chocolat and baguettes, the adventure cost very little. (Check Rail Europe for deals and discounts on high-speed trains to Paris and beyond.)
Here's my hour-by-hour itinerary for a fast and furious, greatest hits guide to the City of Lights:
10:00 a.m. -- Arrive at Gare du Nord
Catch the earliest train you can manage from London. The roughly two hour and twenty minute ride will deliver you to Paris around 9:15 or 10:15 a.m. Head immediately to the line for metro tickets (choose the staffed counter and be sure to have your euros ready - most U.S. credit cards without chip-and-PIN technology won't work at the self-serve kiosks). Or, save yourself a lot of time and purchase your tickets (a 10-ride pack should do nicely for the day) onboard the bar/buffet car of your train.
10:30 a.m. -- Eiffel Tower & Breakfast
There's no time to waste - it's straight to the Eiffel Tower! Hop the metro to the École Militaire stop and walk toward the city's most magical landmark. (Allow yourself a slight detour for that pain au chocolat along the way.) Enjoy your sweet treat in the park, take the requisite selfies and forget the elevator line to the top ... you'll capture another great view of the city before sunset.
12:00 p.m. -- Hôtel des Invalides
Walk to the Hôtel des Invalides, an expansive, palatial complex built in 1671, complete with extensive gardens and a military museum. Its magnificent gold Église du Dôme shelters Napoleon's impressive tomb. While perhaps less famous than other Parisian landmarks, it's a dramatic site well worth a visit.
1:00 p.m. -- The Louvre & Jardin des Tuileries
Next, take the metro to the Palais-Royal stop near The Louvre. Sneak through the Passage Richelieu, which leaves you in front of the great glass pyramid in the Napoleon courtyard. Assuming you've previously visited the Mona Lisa and friends, skip the long entrance line and wander the grand courtyard. Continue your stroll through the lovely Jardin des Tuileries, where you can stop for a café and admire the Arc de Triomphe du Carrousel, cousin to the more famous version down the Champs d'Elysses. (If you'd rather visit your favorite works of art inside the Louvre, begin your day with the museum and then skip the Musée d'Orsay option later in the day.)
2:30 p.m. -- Notre Dame & Lunch on the Seine
Next, it's back on the metro to the Cité stop, which alights on the Île de la Cité in the middle of the Seine. Enjoy the gothic flying buttresses of Notre Dame (if you've already been inside, admire the iconic architecture from outside and skip the long line). Pop into one of the many casual bistros along The Seine for a crepe - or pick up a baguette with some cheese and watch impossibly chic Parisians amble along the river. Then stroll to the romantic Pont des Arts footbridge and admire all of the lovers' locks. (Before it collapses altogether!)
4:00 p.m. -- Musée d'Orsay & St. Germain
Save the Musée d'Orsay for late afternoon, when it's less crowded and you can soak up all of the impressionist beauties. Warning: The museum closes at 6:00 p.m. most days, with the exception of Mondays (when it's closed altogether) and Thursdays (when it stays open later in the evenings), so keep a brisk pace. If you have extra time, take a twirl in the Jardin du Luxembourg and window-shop your way through the St.-Germain-des-Pres quarter.
6:00 p.m. -- Sacré-Cœur & Montmartre
For a spectacular ending to the day, climb to the top of Montmartre and step inside the glorious Sacré-Cœur. You can trek up the steep steps to burn off lunch, or ride the funicular (payable by metro ticket) to the top. Take a rest underneath the white travertine basilica before admiring the scenic views of Paris sprawled before you. Enjoy the sunset with a glass of vin rosé in Montmartre before ...
8:30 p.m. -- Au Revoir, Paris!
It's time to leave. (Quelle horreur!) Head to Gare du Nord and catch the second-to-last train (usually around 9:15 p.m.) back to London's St Pancras International station. Save that very last train in case you need one more beautiful hour in Paris!
* "Paris is always a good idea," though often attributed to Audrey Hepburn, is from the script of the 1995 "Sabrina" remake, rather than the classic starring Ms. Hepburn.
Une dernière chose ...
If you're saving money by skipping hotel stays and fancy restaurants, consider upgrading to the Business Premier class for your jaunt to Paris -- you'll get an abundant breakfast on the way there and dinner, with vino and dessert, on the return, all included in the price of your train ticket.