heavy cream and half-and-half substitute
Cream has a high fat content, fat adds elasticity, flavor and a smooth mouth feel to ice cream. You can use 2% milk in ice cream instead of cream and it will work, however it will not have many of the desirable properties you get from cream. It will be harder and taste "weak" for lack of a better term. 

If you want to make a lower fat ice cream then you can make up for the lack of milk fat somewhat by using more egg yolks and adding stabilizers like xanthan gum, guar gum, etc. See my answer to this question for a bit more detail on gums in ice cream.

It is possible to make a cream substitute by mixing 1/3 cup melted unsalted butter with 3/4 cup milk, but this would only work for recipes where the emulsion of fat in the liquid is not required - all you will get from this is milk with fat floating around in it. This may work in some baking applications, but it won't whip and it won't make ice cream you'd want to eat.