Try 'My brothers place' for roudy beer swilling.
or any of the bars around the Capitol Hill for the 20-30yrs group
M Street in George town for the collage bar scene.
Dress Code: Most bars are casual, any that aren't there aren't worth going to. (DC has enough tension anyway!)
In an University city like this one, there is nothing you can't do, talking about night life.
Also for the more quiet ones, there are cruises that sail down the Potomac and you can have dinner there.
Dress Code: You are in Georgetown!!
You can wear sandals and t-shirt although I wouldn't recommend it, because there might be some nice reastaurants which might not allow you to go in. The bottom line is , you can go very relaxed
Georgetown, Adams-Morgan, downtown
As in most cities, naming a praticular club or bar is ludicrous, as most places only exist for three or four months. The trick is knowing the areas to check out. Georgetown is great for studenty/yuppie-type bars; Adams-Morgan is a bit more grungy/hippie/Greenwich Village-esque; downtown (F&G Streets, 11-13th Ave or so NW) has your large warehouse-sized dance clubs. You can find great Salsa dancing all over town.
Dress Code: Look good. Black is de rigeur. Polyester is recommended, as is leather. DC is a very image-conscious city, so you simply must look fly.