I just recently made a recipe that would work great for you. It's called a ganache.
The ganache I made would be just a little too runny, so I'll alter it here.
3 cups dark-chocolate morsals (semi-sweet)
1 cup cream
1/2 cube unsweetened butter
Combine ingredients in a 2 quart saucepan and heat while stirring with a baloon whisk until silky smooth. Let cool until you can easily tolerate a spoonful fresh from the pot. Gently pour the ganache on the top-center of the cheescake and allow to spread naturaly until it starts driiping down the sides. This is best done on a sheet of waxed paper to minimise any mess.
Add 1/2 cup icing sugar to the remaining ganach, stir to make a chocolate icing and put into a piping bag. After the ganache has completely cooled, decorate with the icing using your favorite tip.
Another great topping for cheesecake is fruit topping cooked and blended. It should be as thick as pie filling, but smooth as silk.
Hope that helps.
Seeeeeya; Goodweed of the North
P.S. I love Nannsi's ideas.