What type of exercises can I do beside sit-ups? They are too strenuous for me.

Abdominal crunches are easier, safer and more effective than sit-ups. Search for a YouTube video for abdominal crunches for the best form.

A good idea is very simple yoga stretching—it’s generally not strenuous, but it keeps your muscles active and it might help you be able to do more strenuous things in the future.