The easiest decorations are non-edible. It's alright if you have to take them off before you cut the cake. No one cares what the cake looks like after it's cut. Fresh flowers are a great way to top off a cake. This past Christmas, I decorated a cake with ornaments. Andrew's graduation was a few days before Christmas, so I combined Christmas and Clemson with these decorations.
I had to make the cake two days in advance, so it worked out really well to pile these ornaments on top of the cake. Instead of stacking the ornaments and hoping they wouldn't roll off, I used a hot glue gun to make them stick together. The topper was also really easy to transport and move since it was all in one piece.
Ribbons are also a great way to decorate. Just wrap a thick ribbon around the circumference of the cake, put some fresh flowers on top and everyone will be talking about your cake.