Asher's one year old cake was a model of his favorite toy at the time, a castle shape sorter (the link is Wal mart just because they had the best picture, not because I encourage you to shop there):
Asher's second cake reveals his obsession with letters and the letter blocks. He chose what letters should be on each block on the cake.
Asher's third birthday cake showed his interest in calendars. Note that his birthday is circled.
Asher's fourth birthday (I guess this retrospective could be called "obsessions through the years") was a map of the world:
Asher's fifth birthday was the trampoline with a kid playing. This is the only one that I don't think represents an obsession although he does love the trampoline.
And the first of Chiara's cakes was her first birthday - a puppy cake based on her love for both puppies and the word puppy.