What are dos and don'ts for a lady in the sac?

Depends on your Dr. You may have limitations from before. It depends on your status and your conditioning and what comorbidities that you have. You can do a lot of exercise, and only be limited by your previous status. Certain medications are also limited and this would depend on your physician.
Communication. Understand your partner, consider using a condom, talk it out so limits are respected, take it slow and easy, stop if there is discomfort or something that is disturbing. But relax and both can enjoy this.