Howdy, The Warrumbungles is big adventure playground. Fantastic if you're (both) very experienced. I can only concur that taking anyone who is not very experienced in as your partner might be less than ideal...maybe you could take a 3rd person so she can have the option to go walking if she is freaked out (it's a very intimidating place even for the more experienced!). The walking there is sensational.
Cornerstone Rib on Crater bluff is great but from the sounds of things it's possibly a bit too hard. I generally have the most fun on adventure climbs when selecting routes about 5-7 grades under my on-sight limit. You might like to consider your girlfriend using this same scale...you'll feel happy and in control rather than totally freaked out. Climbing there is pretty wild.
There are a bunch of other areas probably closer to home that would perhaps be much better for you and your girlfriend.
Definitely weather is something to consider as it's horrible when it's too hot.
Another word of wisdom is to treat the water at Balor Hut (there is a big tank). I did this every trip but my last one and paid the price (about 3 days of not being able to climb and feeling totally horrid, vomiting etc). If I can spare you (and your girlfriend) that pain and disappointment it would do me some kind of satisfaction.
Here is something I've been working on. If you do go (or anyone else has some good photos of Bluff Mountain in particular) please let me know as I'm keen to improve this free guide! I hope you find it useful. There is also a (very old) rock guide which you could buy from Wild/Rock magazine (issue 20, 1994). http://www.rock.com.au/