There’s only one route for Cader, straight up and down the same track (unless you are naughty and take the footpath (Pony Path) down) – always thought that would be a nice route but it will put you down into Dolgellau the wrong side of a bl**dy great hill!
The run down from the peak of Cader is (IMHO as an XC mincer) a cracking descent.
If you do go down the Pony Path there is a car park at the base – head on from there past campsite and towards the farms and there are some good tracks there, a loop could be made if you don’t mind some cheeky.