DH wouldn't let me get DD christened (fair enough, we only go to church for weddings and funerals, but I'm still a sucker for 'pointless' tradition), but I remember photos of my christening cake. It was pretty much a traditional wedding-style cake - fruit cake (apparently de rigeur for celebrations in the days before mud cake) covered with marzipan icing, and it just had something like my name and the date, and mum still has the little cut-crystal crib with a plastic baby in it that was the decoration for the cake.

I'd recommend getting it done professionally, either at a place that specialises in wedding cakes etc, or just a slab cake made up by your local Woolworths/Safeway. Actually, I can't recommend the Cheesecake Shop highly enough after the fantastic consultation and cake I got done up for my wedding, they are more than happy to discuss options with you and go through folios of celebration cakes they've done in the store, and help you out with a quote so you can decide what you want vs what you want to spend. They do so many different styles, so many flavours and so many decoration options, I'm sure they will be able to help you out.

Hope the christening goes well, good luck with the cake