Have a look at the Bikefax guidebook as well, loads of great riding round there.
From Ogwen, there’s a nice circuit round Moel Y Ci and through Dinorwic quarry, as mentioned above.
Conwy mountain is a bit of a cracker too.
Loads more threads with specific details if you hunt around a bit.
Oh, and if you’ve got newer riders with you, don’t bother with the Llyn Colwyd/Llyn Crafnant route that someone will suggest shortly.
The less experienced members of your party will lynch you, before throwing their bikes and your still twitching corpse into the depths of Llyn Idwal.
Not an easy route by any means.