Planks are a great exercise that targets the abdominals while in spine extension.  
 
Here is a video demonstration of different variations of planks. You can do a low plank or forearm plank, a high plank or side planks. If you need to modify it, you can drop to your knees. Remember to keep the abs tight, shoulders down and back and elbows directly under the shoulders. You can start with 20-30 seconds and work your way up from there.