and of course as has been said the Mach loop. Also coed-y-brenin is a great place if you want to do some mountain biking. .
The Mach Loop?
Anyway I'm from south Wales but I recommend the north. Been going there for years by motorcycle but have lost interest due to speed cameras and more bastard cars. Recently done some cycling (rented) around Llanberys. Done the Llanberis pass many times but earlier this year it was a first for me to do it by push bike. ****ing breathtaking! Even better pedalling than by 1000cc motors.
Theres also a MTB trail next to Betws y Coed which I thoroughly enjoyed. Its also where I had my best accident. 2 paid weeks off work with a toe fracture and I could still drink beer! (altho not much else).