Mash is going to be the best option, but you can vary the kind of mash...

If you don't like potatoe, you could try mashed parsnips, swedes, sweet potatoes, turnip, etc.- you could even experiment with things like artichoke mousse or houmous- although that might not have the required consistency.

Generally though, the whiter the better- then you can do wonders with a bit of food colouring & (for example) have three tiers in baby blue, pastel pink & lime green, with wonderful 'traditional' decorations on top... much more of a surprise than the T-bone steak on top...