If you don't want to use ice cream in your milkshake, you could use just ice and a little extra milk in your recipe, but unless you're lactose intolerant, I wouldn't recommend it. Besides, there's plenty of dairy-free ice cream options on the market today! If you're out of ice cream and don't feel like heading out to the store for replenishments, it's worth a shot, but otherwise, I wouldn't recommend this substitution.