Thinking about making a 21st birthday cake for my daughter who collects key chains. Any ideas?

Hi,
Why not make simple cake base, could be a basket shaped one or even just a sheet cake and go to town on the decorating of the town, make loads of gumpaste/fondant models of her key chains and decorate the cake with them?

or it could be one massive cake replica of her favourite key chain?

Hope this helps, good luck!

Yep, a pile of her favorite key-chains. You may be able to use edible images for some of them. Or make a small, very small (like 4" tall by 4" round...something like that) 2 tier cake decorated in her favorite colors and attach a large chain and ring to it, so it's a big cake key-chain! You could put it on a base cake if you are needing to serve more than just her.