If you do BYP, there's another few nearby to make a day of it (on top of C&J mentioned by davel) BYP opencast with slope to some underground stuff, Hafod Both (further towards Rhyd) and Pant y Wrach Deep and Middle adits. You'll be stuck on these levels wthout Srt though. There's another little level in Cae Fali that goes under the FR and pops out nearby it. if you've still not had enough.
These were open when I did them, but it was a while...
Then there's the Llanfrothen Arms for refreshments aferwards.