Nevercan, I have a trampoline to to make in 2 weeks time, complete with a girl jumping on it and stars, its a square cake, that looks OK, Google it and theres one on there with grass and some flowers thats the one.
I always get my ideas for a specific theme from Google and pin interest, even if it doesn't help by seeing how to do something it gives me an idea to copy. You could make a mini trampoline out of fondant.
Have you googled 'gymnastic theme cakes'? Will give you lots of ideas. Ebay is good for cake decorations. Photo toppers are also good if you want to keep it simple and then add gymnastic themed decorations to it.
I am thinking about making a birthday cake for my dd's 3 and 5 joint bitrthday party. Any simple suggestions for decorating the cake with a gymnastic theme or where I can get cake decorations in this theme?