Myself and Bugs did some bouldering at the Cavern last year. The rock isn't all great - I took a nasty fall on a really easy rock (about E-grade) when my foothold broke off. Bugs took a few nasty falls on top rope on the one face of "Blockhead" (which was the best rock we found - it has 3 good faces and 1 terrible face). So just be careful. Full details and GPS co-ords at:http://www.vertical-endeavour.com/forum ... avern.html
I know there are spots near Mahai where people set up top ropes to abseil and boulder. I don't have further details though.
If you go to the Crack or the Mudslide there are chains and chain ladders on those - so easy to set up a top rope (although I don't know if there is anything worth climbing there).
Have fun - its an awesome spot
Btw. I highly recommend the gorge walk as a day out by the way (if you haven't done it before). Keep going past the chain ladders if you want a really good day out