You are either game, or a really good baker! I did this cake for my dd's first birthday, and it almost got the better of me! It took me 13 hours to get one perfect bottom, and one near perfect top.

So I would prob say try the marble first, and see how you go with that, and then maybe do rainbow icing. I iced the top, and then for the bottom, I made a chocolate mould out of the cake tin for it to sit in, and it looked fabulous, even of I do say so myself ;P

Good luck, and I'm sure your DD will love whatever you make for her x