How can you work out with a fractured knee?

It's been about 5 months since I fractured my knee... Luckly didn't need surgery! But I had my baby in march.. And it's been hard to loose the baby weight.. And now my husband wants to try for our 4th (last one after that..) 
Got an okay from the doctor... But I want to start working out in what ever I can to not gain more weight... any tips ?