Some things you can do with a ruined cake are make smaller cakes, make cake pops or cake balls or even add the ruined cake into some pudding and make cake based pudding.
You can also use a ruined cake to make cake crumbs or cake croutons.
Another thing you can do with a ruined cake is to turn the ruined cake into tres leches cake.
Or you can simply eat the ruined cake as just because the cake has not turned out good like you hoped and wanted the cake to turn out it does not mean the cake is bad to eat.
If I've ruined a cake when baking it I just put it in a bowl and then still eat it like normal.
I make the cake crumbly and then put icing on the ruined cake and still eat the ruined cake.
There's nothing wrong with just eating a ruined cake although it may not be as enjoyable as it would be if it had not been ruined.