Go retro with anti-E-readers. Anyone can buy a Kindle or iPad for a SO, but you're better than that; you're not a lemming. So go retro by bringing home a copy of the Washington Post Express and Washington Examiner. If your SO's a newshound and has a political lean, only bring one of them home lest you want another fight with your SO.
Soon an entire generation won't know which came first.
Create a tapas dinner. Though Metro's no food or drink policy has been around a long time, that shouldn't stop you from providing a grand tapas meal. Go on and grab the partially opened bag of Doritos on the window pane, the box of Sour Patch Kids from the floor, and the other half of that Otis Spunkmeyer chocolate chip cookie. If you're lucky, you might find some McDonald's chicken nuggets at the top of the escalator on your way out. Your SO would appreciate them even more if they're still in the box. Don't forget to wash things down with the almost empty bottle of Mountain Dew rolling down the aisle.
Grab a seat toward the back for optimal drink and food opportunities.
Provide chauffeur limousine service. If your SO is always asking for rides to and from the station, around town, or to the airport, grab some bus schedule pamphlets. They're great reading material after the retro E-readers and your SO won't have to bum rides off of you. When asked if you can give a ride to BWI, just say, "there's a bus route for that". If the SO is a world traveler, grab some MARC train schedules too. It's the gift that keeps on giving (you back your free time). SOs everywhere will be driven where they want, sort of when they want, and in vehicles that they may confuse for limousines if they're hallucinating.
Imply you want a future together. Near the bus pamphlets, grab a flyer warning you of future Metro repair delays and escalator outages. Earn bonus points by bringing home road construction public meeting notices too. These show that you're thinking about the future with your SO; at least how it'll impact your commutes. Nobody appreciates foresight and long-term relationship planning more than your SO.
Create a homemade romantic card. In this age of Twitter, you must be brief with your words. Pickup a Metro card from the ground and write something sweet on it using 15 characters or less depending on your handwriting size. "I love you" is only 10 characters, but "I don't care about our relationship enough to remember this day" is just too long and honest. Don't have a pen? Borrow one from the station manager or grab one from just below the third rail; I've heard there are some great Montblancs down there.
No SO can be upset when your card is covered with pandas.
Listen to live music - Plan A. Take your SO on a Metro ride around 9 am on a weekday, staying between Metro Center and Gallery Place. This ensures you'll overhear music playing on several incessantly loud iPods. If you don't like the genre, move to another car until you find one that sets the romantic mood. Keep changing trains to keep the musical jackpot surprises coming.
Listen to live music - Plan B. If your SO is picking you up from the station, have them park the car and walk back to the station entrance to take in the local and live music scene. Undoubtedly, you won't find great musicians, but you might find the 5% that are bearable. If anything, you'll be hip to the area's up and coming guitarists, paint bucket drummers, and Peruvian flutists.
Buy a $5 bouquet of flowers. Flowers are a great sign that you care, even the wilted ones from the flowerseller outside the station. At $5 for a bouquet, you'll get credit for caring and being fiscally responsible by not buying from a local florist whose flowers are needlessly arranged well, tasteful, and better still - alive.
Bring home this Diego Rivera painting called "The Flower Seller" and you'll really make your SO happy.
Take the SO to an amusement park. If you follow Plan A to Listen to Live Music, remind your SO that a Metro ride doubles as a rollercoaster. Make sure you're both standing up and see who gets sick last from a herky-jerky manually automated ride. Maybe you want to impress by not grabbing a handle for balance - how athletic! It's a fun experience that's all included in the price of admission.
Just remember Valentine's day next year. If these don't match your SO's lofty expectations, then remind them that it's better than last year's gift - a jar of belly lint.