Wow, climbing Snowdon twice in a day would be big. Rhyd Ddu would be a very tech descent, must be pretty much a 100% push up?
Cadair Idris is good, but a case of after the lord mayors show maybe?
Capel Curig north towards Ogwen on the old coach track, turn right over the A5 and climb up to that lake with the unpronouncable name, drop down to Tredriw, back up to the more easterly of the 2 lakes, along the shore, climb up again and join a fun descent to Capel Curig. Sorry, bad description but good ride.
Pont Scethin is a good day out too.
There must be loads more, but you never hear as much about natural riding in Snowdonia as you do for the Lakes – dominated by the trail centres I guess