Cindy,
This one is super easy and is refreshing too.

1 large (12 oz.) can frozen OJ
1 cup sugar
1 2-liter bottle of chilled Sprite

Partially thaw the Oj (do not add water) and stir in the sugar. Add the Sprite, a little at a time at first, stirring to help disolve the sugar.

I have had to stretch this recipe in the past by using even a little more Sprite, and it was still good, just not quite as strong. Though I've not tried it, I think that pineapple sherbet would be good floated on this, especially if you did add more Sprite to help cut the added sweetness from the sherbet. Good luck.
Good luck.