In reply to Comby86: There aren't many good top-roping venues in Snowdonia. Some of the better ones for a 1/2 day have already been mentioned.
Clogwyn Cyrau is also a potential option
http://www.ukclimbing.com/logbook/crag.php?id=872 especially if you are staying near Betws, but it has a longer walk-in (and potential to get lost in the forest).
It is also worth considering the slate quarries - there are now a good number of really easy routes around, although it is worth bearing in mind that the rock quality (and climbing quality) is fairly variable in the lowest grades. Australia
http://www.ukclimbing.com/logbook/crag.php?id=10 is the obvious quarry with a 3, a few 4s and a good number of 5s but it is worth checking the UKC database or the slate wiki
http://slate.wetpaint.com/ for some of the others as well.