Not too many campsite near Brechfa from what I can tell.
Cwm Rhaeder is even further away, but I've heard good things about it.
Pick a good day and do the Black Mountains killer loop (I'd go anti clockwise next time, but that's becuase I want to ride down Y Das soon).
There's a good loop from Bwlch too, techy descent down to the base of Mynydd Troed and around the back of it, pick a dry day.
Gap is a classic - someone will come along in a moment and say it's over rated, but it's classic xc stuff. You get up high, proper mountain feeling and a long descent from 600m up. There's a few variations that improve it slightly , IMO. However, the "best" bit has been flattened out by several tons of gravel, which might increase you chance of running into a fleet of lost Waitrose shoppers in their pretend 4x4s.
There some stuff on the Black Mountain that I'm very interested in, including a long drop to near the Dan Yr Ogof caves, but like Brechfa, it's a bit out of the way.