If I bake a cake , but I'm not going to decorate right away, what can I do to keep it fresh?
Double wrap it in plastic wrap and freeze it.
What type of cake?
If you use a box mix i wouldn't worry about the cake going dry after a few days..mine have tasted just as moist a week later. I would just wrap it well with saran wrap until you decorate it or freeze it.
If you bake by scratch i think this is more problematic, keeping it moist tasting after a long time. But from what i've read people use simple syrups and such to add moisture to a scratch cake. HTH