As Simon suggests there's plenty of room - wildcamps are limited only by your imagination. Look for flat areas between the peaks, preferably with a water source close by - but don't pollute it.
I've left my car at the free parking places alongside Llyn Ogwen without problems, also there's a layby between Nant Peris and Pen y Pass that's often used by Welsh 3000ers, and another free car park near Rowen, north of Foel-Fras (the last 3000 foot top of the Carneddau.
Lots of good routes. For rocky terrain head up Tryfan, Glyders Fach and Fawr, and Y Garn (with a missable scramble along Bristly Ridge up to Glyder Fach) or miss Y Garn dropping down the fantastic path through the Devil's Kitchen. For more rolling country go for the Carneddau.