So glad you all enjoyed Wales! I was brought up on the border( Chester) and my mum's family has welsh as a first language. It's one of my favourite places in the world so I hope you don't mind if I add a bit from a "locals" perspective.
The language- good luck trying to speak it! :-) lovely to listen to, ugly when written down. All children in Wales now have to learn it in school but historically it was banned from the education system. All teaching was in English and children were punished if they spoke it in class. My taid ( welsh for grandad pronounced "tide") was certainly beaten for speaking it. Today it is more commonly spoken as a first language in rural areas and in north Wales rather than south but all signage and official communication must be in both languages. One of my favourite aspects of the language is how the welsh language has adopted and adapted " English " words to their own language. Look out for the signs for "tacsi/taxi". You might also enjoy this site, good luck with the pronunciation!
Snowdon- you don't have to walk up it. There is a lovely little railway that can take you up and down the mountain. http://www.snowdonrailway.co.uk At one point of the trip the train goes through an exposed stretch that gets quite windy. In the days when everyone wore hats, even to go up a mountain, many used to blow off and down the slopes. The rumour is that local kids used to wait to catch them and then sell them back to the visitors when they came back down. No idea if that is true, but knowing many north waleans it seems likely.
Betws-y-coed. It is quite a touristy place I think because it is an easy trip from the coast where many of the tourists stay. If you want another side to its history at the start of World War Two many children from Liverpool were evacuated to the area. If you are not aware of this bit of British history it is worth learning more. I don't think it would happen today! My dad was evacuated a week after his 7th birthday, leaving behind his parents and 2 younger brothers. He was billeted with an old woman who wasn't very kind to him. The family story is that my nan saved the whole family sweet ration to send him a bars of chocolate but the old woman stole them for months. One day my dad met the postman who gave him his chocolate which my dad promptly ate and then vomited all over the woman's sofa! Kharma I say! On a more positive note my dad was soon fluent in welsh which served him well when he married my mum. Her family was not happy about her marrying "out" and weren't initially very polite about him in welsh, at family gatherings! One day my dad accidentally responded to a question in welsh and the cat was out of the bag
I agree the slate mines aren't beautiful but they have their own grandeur I think? It was a very different form of mining to the coal mines in South Wales. It is also much harder to disguise the slate tips than it is to disguise coal slag heaps. Plants grow much easier on coal heaps rather than slate. There was also a concerted effort to maintain slag heaps in South Wales after the Aberfan disaster in the 1966. If you are not aware of this event, be warned you might cry.
When they closed the slate mine at Dinorwic they converted part of it into a hydro electric plant which is amazing http://www.fhc.co.uk/dinorwig.htm. As my family was "local" we got a guided tour of it when it was being built. I will never forget the size of the turbine halls that were created by men with picks and shovels, and a little bit of dinamite.