Buttercream melts in hot weather, I had a cake where all the icing slid off the sides one time. Fondant works better, but if left too long will begin to melt or absorb moisture. I think the humidity got to one I left out for an extended time in the heat.

Also, as a filling, you may want to crush oreo cookies, mix with (I used buttercream) and use as a filling, this was a hit with a group last spring. Most did not like the filling made with vanilla cookies. Good Luck