I’m still on the hunt for good, rideable mountains in Snowdonia. Cadair Idris is one, though it is much tamer than Snowdon. After that I’m stumped.
There is at least one footpath descent off Snowdon that’s the business, but last time I named it in here I got a bollocking over email from one of the local bike advocacy guys so you’ll have to guess I’m afraid.
As for the lakes, most of ir_bandito’s threads are good, the Putting the Mountain Back Into Mountain Biking thread by Danny and there’s plenty of ideas in my annual mountain thread (with more to come in the next couple of weeks).
http://singletrackworld.com/forum/topic/2015-a-year-in-mountains