I have one in my photos if you want to take a look, it's pretty much the emerald city and dorothy's shoes with the yellow brick road..HTH
sheet cake or tiered? let me know and i can probably think of some ideas. 
I have one in my photos if you want to take a look. Pretty simple - round cake, fondant yellow brick road, chocolate transfer rainbow, buttercream poppies - although the mom did supply the figures...
At least maybe it'll give you an idea.
HTH
ok lets see.
for tiered you could do a yellow brick road going up from the bottom to the second tier.
then for the top tier you could have it be emerald city. for a topper, if you want one, you could do either a throne or dorothys shoes. for borders around each tier, you could do fondant flowers and on the bottom tier, where the road starts, you could have the flowers continue up the sides of it. does that make any sense at all? i'm not too good at explaining things. 
or you could do a border with the grass tip around each tier.
for a sheet cake you could adapt that idea.
HTH
oh meant to say- im assuming it can be 3 tiered. lol of course you can adjust.
I thought this one came out really nice as a sheet cake.
http://www.cakecentral.com/cake-photo-1231825.html
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