If fish/shellfish are possibilities, then Ray's the Classics is by far the best restaurant in Silver Spring. It is primarily a steakhouse, but their fish and crab dishes are fantastic. They do have one vegetarian entree that is on and off the menu (a portobello/eggplant tower), but if you let them know when you make the reservation, they will be sure to have it.
Ray's the Classics
8606 Colesville Rd, Silver Spring, MD 20910
Seconding most of the above, especially Mandalay, which has an extensive vegetarian selection. There are three Ethiopian restaurants in downtown Silver Spring all of which have strong veg options--Abol is our favorite (on Colesville across from the AFI). Ghar-E-kabab has plenty of veggie options (and the best aloo gobi I've ever had), as does local chain Lebanese Taverna located in the mall-ish section of downtown Silver Spring.
For more traditional options, Adega cafe on Ellsworth has a few nice veggie dishes and The Quarry house serves up veggie burgers, sandwiches, and hummus plates along with their tater tots.
I agree with the earlier poster, your best options are ethnic foods. It depends on what kind you like and how far you want to travel. I'm assuming you are staying at one of the downtown Silver Spring hotels.
Lotus Cafe is a nice Vietnamese place, but it is about 3/4 mile from the downtown SS area along a rather unpleasant walk on Georgia.
Mandalay is excellent & a little more exotic. It's Burmese - sort of a cross between Chinese and Indian. They have a whole page of vegetarian options & several more of meat ones.
Other places to consider
Mi Rancho - for Tex-Mex. Very festive atmosphere. Eat on the patio if it's nice.
Asian Bistro - in the center of SS. Chinese & sushi with lots of vegetarian options.
da Marco - for classic Italian. Stick with the pastas here.
Ghar E Kabab nice little place with Indian/Nepalese food.
Have a great trip.
MI Rancho MD
8701 Ramsey Ave, Silver Spring, MD
Da Marco Italian Gourmet
8662 Colesville Rd, Silver Spring, MD 20910
8537 Georgia Ave, Silver Spring, MD 20910
944 Wayne Ave, Silver Spring, MD 20910
Mandalay is a Burmese (sort of the love child of Thai and Indian food) restaurant and it has great vegetarian options, their salads and noodle dishes are really good. Mandalay has a great track record for modifying their food, they will happily substitute soy sauce for fish sauce in any dish. They also modify the spice level. http://www.mandalayrestaurantcafe.net/
Near downtown Silver Spring, Lotus Cafe at 8077 Georgia Ave has a good vegetarian selection. You can also check out Mandalay at 930 Bonifant Street. Both are a short walk from the Metro and have street and public parking.
Mandalay Restaurant & Cafe
930 Bonifant St, Silver Spring, MD 20910
8073 Georgia Ave, Silver Spring, MD