I need to advice on exercises for loose skin/abs!

I lost a great deal of weight over the past year and half,so I'm looking to tone up my body now. I have a lot of loose around my stomach my belly button is practically sinking in.What exercises would you suggest to tighten the skin and surrounding abs? I already do crunches,bicycle crunches,planking,side twists.I just need advice,and help :]