I second the Nant y Arian idea, perfect for beginners as the single track is fun but not scary. No showers though (unless they have built some in the last 6 months), which is a downer this time of year.
Marin is a good idea. Will have a “great outdoors” feeling to it this time of year, with snow capped mountains in the distance. She will like ByC for it’s food and hotels/B&B’s. Then if she’s keen for more on Sunday you can do Penmachno. EveryoneÃs a winner!
#Do not go to the Chilterns this time of year, the mud, horses, punctures, dog poo will put her off for life#