I’ve done some research on a Music Together location in the Washington, D.C. / Bethesda, Maryland area and I think I’ve decided upon Little Steps Music.
As described on the HealthyBot website, Music Together is a great activity for kids in the winter, but it can be a good activity any time of the year.
From what I’ve heard, registration is in high demand, so I’m planning to go early to sign up. They haven’t posted the dates yet, so I’ll have to keep visiting the website until they do.
UPDATE: Read my thoughts about Music Together at HealthyTot.com.
Check out Kindermusik at www.kindermusik.com, too - there might be an open class that's a good match for what you're looking for.