the easiest path is the Llanberis path, and generally follows the course of the railway back to its start point. The walk is described here:-
http://www.eryri-npa....-Walks/llanberis-path
Over 30 years since I last did it but eminently walkable and follows the general line of the railway back down. The path is wide and over rocks in the upper part, but not difficult either.
try the pig track, this is a very popular route. Some of my mates went on the Crib goch route. When we arrived at the top they took the p1ss, saying i went on the mother and toddler route. The route i took should take no more than three hours.