Sit ups wouldn't be good to do for a few weeks yet.
From memory the physio I saw said to do gentle pelvic tilts and knee rolls (laying on your back and keeping your knees together and rolling them from side to side) as well as some gentle pelvic floor squeezes.
Also in crawling position gently arch your back like a cat.
Remember your ligaments are still very soft and doing sit ups can do damage to your pelvic joints and back, not just to your stiched up bits.
I wouldn't attempt to do anythong like a traditional sit up or weight bearing excercise till after your 6 week check up.