So let’s review a huge list of abdominal exercise substitutions:

1) Replace crunches with:

Planks, mountain climbers, and stability ball jackknives

2) Replace cross-crunches (i.e. twisting or oblique crunches) with:

Side planks and cross-body mountain climbers

3) Replace sit-ups with:

Stability ball jackknives, stability ball planks, stability ball pikes, stability ball rollouts, and hanging knee raises.

via 10 ab exercises to do (print this out) – Early To Rise.