In reply to Scarab9: The Tarren loop from Abergynolwyn is good, going up from Dolgoch through the waterfalls is nice. If you like your climbing, that might be one for a run as well - over a 1000m of ascent in about 16Km or so as you start off from something like 40m! If the train is running, you can even do it with a steam powered start/finish from Tywyn.
South end of the Rhinogs - did a loop around the S and SW ridge of Y Garn, which is nice and quiet and especially good for admirers of stone walling
Good views of the rest of the Rhinogs and all the Snowdon hills from a different perspective. Heaps of other stuff there as well.
Traverse of the Dyfi valley hills from the top of the pass on the Corris-Cross Foxes road heading east to Maesglasau and coming down the side of the valley with the big waterfall
Arans from Cwm Cywarch or the traverse if you have two cars.