A common idea for an over-the-hill cake is one in which the cake is a hill. Other common ideas involve graveyards, rocking chairs, and either a large number of candles or a small number of large candles or even a question mark candle.
An over-the-hill cake is a cake celebrating either the 40th or the 50th birthday of an individual, depending on the local idea of when one is officially over-the-hill. The term plays on the idea that life is all downhill from here.
Things that are stereotypical of old people are common items to depict with an over-the-hill cake, such as glasses or hearing aids or walkers. There are many places online to find picture ideas for cakes, such as Coolest Birthday Cakes, Wilton, and Photobucket.