Cake

Ok. So here's what I think would be the simplest and most elegant. Especially since you two are going with a black dress and a white dress (which is very chic)

Buttercream frosted chocolate cake, black satin ribbon

White frosting, black satin ribbon.

But yours will be 3 tiers:

and we can add flowers (imagine that these are blue, not white):

 

Considering that this is just a white cake, too, and not something too crazy, we could probably do a fondant (like the center picture) that we could easily make out of marshmallows.

What do you think for cake toppers?

If we do just a simple white cake with black ribbon at the bottom of each tier, and some of your blue flowers, I think it would be very elegant to do the cake topper as two love birds.

or blue birds.

 

Or Blue Love Birds.

these can be painted your shade of blue

Then we have some built in themes for centerpieces/ favors:

Like this from Martha Stewart. But these could be done as part of your centerpieces, rather than having one at each place setting. 

Even if you don't do the nests, I like the birds : ) --mk