What are some easy things you incorporate (that I can copy) that keep you more fit?

I'm skinny fat in my opinion... I'm 5'5 and about 128 or 129lbs. But I don't have much muscle, my stomach is loose from two kids, my butt isn't perky, my arms have no definition, etc.

I also don't have much time. My kids don't love naps and I stay home (no daycare/babysitters). I joined the gym but never went.

Running? I have this vision that if I can get out for a run 3 times a week and incorporate some sit ups/push-ups/ squats into my day I'll look good. Tips?

I don't need to lose weight, just be a bit more active to keep things in check, if that makes sense, and I'm wondering the best way to do that.

ETA: I know I can't "spot treat" a certain area. But just looking for a routine that is easy to incorporate into a busy life. I've found work out plans (30 day shred etc) hard to stick with.