In reply to SidH:
Ordinary route on Idwal is good in the rain, potentially you could continue to the Cneifion Arete but I can never be bothered if the cloud's down.
And of course there's Lockwood's Chimney. Or Bryant's Gully which is more of a scramble than a climb, but great fun when in spate - worth missing out the bit just above the big chockstome if you're on your own as it's pretty loose.