In reply to abcdef:
Loads of interesting walks in Gwydyr Forest, depending on desired distance. You can walk to Llyn y Parc or Llyn Elsi from Betws, or up to Miners' Bridge and back. Llyn Crafnant and Llyn Geirionydd are both worth a saunter. Fairy Glen is impressive but you have to pay to get in, and the same goes for Conwy Falls. There are so many lakes and mines (Hafna is good and has a car park) in the forest that exploring can be quite rewarding.
Slightly further afield but Cwm Penamnen (south of Dolwyddelan) is surprisingly lovely. You can walk down the road (single track to nowhere) or along forest tracks. Another option for something different would be to drive onto the Migneint and either walk along the track to Llyn Conwy or take the boggy path to Llynnau Gamalt - both very bleak but wild and remote on a good day.
If you got a good day and wanted a "mountain" than Clogwyn Mawr above Capel is a fine and rocky viewpoint. Approach from the west.