Well, what do you feel comfortable doing? If sheet cakes are what you are best at, do that! I don't do a whole lot of sheet cakes, so I'm bad a designing them.

But, if you don't mind doing some tiers, how about a 3 tier? You could do each tier at a different height theme (ground, buildings, sky) and then have a little gumpaste airplane on a wire sticking out of the side of the top tier so that it looks like it's flying in the air! I know...I tend to go overboard sometimes...

But I think it would be cute!!
Plane here!
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Good luck! Let us know what you decide!! Oh...and WELCOME!!!