Best lower tummy workouts?

Jené • 23 years old married, mommy to a beautiful daughter 12-22-13 and handsome son 9-8-15.💜💙
Next Tuesday I'm 6 weeks postpartum an his my second baby. I love my new little man an I had body issues before my first and after that I stopped caring but I do want to just tighten it up my lower tummy just a tiny bit. Any workouts you guys find work the best? I am breastfeeding so that's helping the pounds shed off a bit.