Yoga is an excellent idea! It's wonderful your obstetrician suggested it - prenatal yoga classes are a great way to stay active and mindful of your body during pregnancy. I've also heard good things about pelvic floor physiotherapy, especially for preparation during the third trimester and for postpartum recovery. Some women in my family swear by aquatic exercises too, like swimming or even just walking in shallow water - it's gentle on the joints and a full-body workout with the resistance of the water.
It's also a great way to meet other moms-to-be and bond with your bump! I'll definitely be joining some prenatal yoga sessions soon