Prenatal yoga is fun (: Other than that, walking and swimming like you mentioned. How much activity you can do all depends how active you were before. And cocoa butter for the stretch marks ! Start using it right away, because I didn't think it was necessary until I was bigger and then around 17 Weeks I woke up with massive stretch marks down my side. Needle
Needless to say, I bought some Cocos butter immediately and now slather it in every night.
You can't prevent getting stretch marks. Its either you get them or you dont. Once you get stretch marks you can only reduce the appearance of them.