N D says:

Interesting tidbit, but the external obliques actally fire more during a pushup than they do a crunch. Depending on what part of the abdominals you wish to work, there are several different strategies you can employ. The internal obliques and rectus abdominus fire during crunches, and they are a valid exercise for that purpose, but the external obliques are better served during stabilization of the body during limb movement.

Intensifying abdominal exercise is largely related to lengthening the lever arm. As an example of a progression for this, I will use your hated crunches because it is a straightforward example. If crunches with the arms crossed become too easy, the arms can be moved to the head, and then above the head, then adding weight, etc. These same principles can be applied to exercises like bridging and planks (both of which are excellent isometric abdominal exercises)

The nice thing about planks and bridges is that you can set the intensity through duration and creation of an element of instability in the exercise, either through a ball, a wobble board, or if you are really lucky, a vibration platform. Movement of the limbs during isometrics also challenges the abdominals to maintain stability, which is their most important function.