Its does not get any gentler than the Snowdon Ranger for your return route.
The path is very gentle for the first half mile or two, totally unlike any other path on Snowdon and then a little steep in places, but still very easy walking, and then the rest is on grass back to the Youth Hostel.
Ive never walked up the Rhydd DDu and come down the Ranger path, but both paths are less than five miles from Beddgeleret, with the Rhydd Ddu being only three miles from Beddgelert itself.
All the best.