Go!
There are so many good indoor things to do -
-down a slate mine in Llanberis or Blaenau Ffestiniog,
-down a copper mine around Beddgelert,
-soft play centre in Caernarfon,
-woollen mill at Llanwrst (free ad mission) where you can have a go on the loom,
-swimming at Porthmadog and Harlech
-explore the electric mountain at Port Dinorwig,
-the alternative centre for technology in Machynlleth,
-a steam train ride on the welsh mountain railway or ffestiniog
and tons of other stuff too -
best of all take a sowester and some wellies and stand on the side of a hillside and just breathe in! nothing like it.