I'm being presumptous(sp.) here, but I vote that you try for variety rather than increased intensity. I know nearly every story (including my own) starts slowly and gets more and more intense. Eventually, you're left with the problem you now face -- how can it possibly get any "harder" without killing the poor girl?

Try doing some things that are different WITHOUT being any (or much) more intense.

If your question was really, "What different can we do?", uhm...I dunno.