I run and there are core excercises you can do. Look up ab excercises during pregnancy.
Walk,swimming, squats, jogging(if use to it), exercise ball(I have one its amazing look up birthing ball exercises on YouTube).
It greatly depends on what your fitness level was before you got pregnant and what type of exercises you did. It's best to ask your doctor if you can continue a specific routine, however walking and swimming are supposed to be good for almost everyone.