Dream Whip works better if you want a whipped frosting. And it's stable enough to decorate or or with.
You can make it with milk, cream, or water if you are lactose intolerant.
Real whipped cream will work as well, but you have to keep the cake cool. When whipping the cream, just add 2 tablespoons of instant vanilla pudding to it. The pudding will stabilize the whipped cream so it is workable for decoration and to frost a cake with. (2 tablespoons per pint of whipping cream).
If you live in a city, you can usually get some BetterCreme from a grocery store bakery. You can get it in liquid form from them and whip it at home. This is what they use for cakes where people don't like buttercream on them.
Be warned though, its very sweet.
You could also use a stiff meringue. There are some good recipes for stiff meringues for decorating, this too would hold up on a cake. You can use a Brulee torch on it so you dont have to put the whole thing in the oven.