Tourism down south - suggestions please

Tunisia holds a special place in my heart. My father grew up in Tunis around the 2nd world war then came to live in Italy and as a kid I came there many times in summer in the 60's and 70's with him.

I came back a few times in the 80's traveling around with my wife and mother and really loved it.

Now after so many years without I am a little scared of going back and finding that my memories do not exist anyomre, the kindness of people, the super great food, the quiet of many places.

I am thinking about organizing a new trip to find out and would like to visit places I stil do not know down sourh: the Kerkenna' islands and the great erg dunes in the interior.

Would Sfax be a good base for both? Any recommendations on a goo local travel agency to organize things?