I only bake for family and friends and I have a wedding cake coming up with lots of small fondant accents/decorations. Super Mario Brothers theme so think lots of coins and blocks and pipes. I want to take my time on these so I can really get a lot of detail, but how do I keep the fondant pilable so it won't dry out before I attach it to the cake? How far ahead can I start working on them?
Any tips appreciated.

The fondant will not stay pliable for long especially if it is thin - small accents will dry out within a matter of hours quicker depending on size and thickness. My advice would be any accents that have to be attached directly to the cake need to be made and attached roughly at the same time otherwise they won't glue/sit flush with the cake and will snap if you try to bend them - more so for accents going around a round cake. If however eg. coins are going to be piled on each other on the top of the cake they won't need to sit perfectly flush so they could be made in advance - I suppose it depends what you decide to do. Not sure that this is much help but i'm just trying to avoid you spending time making stuff that won't attach properly to your cake.