Re: What kind of demonstrations could I do with potassium permanganate?

A great many! But why are you trying to use it up? Potassium Permanganate
can be kept from year to year, allowing you to repeat your demonstration.

Anyway, there is a series of books put out by the American Chemical Society
which list numerous demonstrations that are "tried and true". There are an
excellent source book and have some experiments that use KMnO4. They are:

Borgford, Christie L., 1943-
     Chemical activities / Christie L. Borgford, Lee R. Summerlin.
     Washington, DC : American Chemical Society, 1988.

Summerlin, Lee R.
     Chemical demonstrations : a sourcebook for teachers / Lee R.
     Summerlin, James L. Ealy, Jr. 
     Washington, DC : American Chemical Society, 1988.
     (two volume set)

A copy should be available at a local University library or directly
through the ACS.

However, short of looking experiments up, and off the top of my head:
KMnO4 is a powerful oxidizing agent, it can be used to oxidize double 
bonds. Hence, it could be used as a test for, say, saturation in 
margarines. Titrating a double bond with permanganate will give the 
degree of saturation. I haven't tried this one but it should be relatively
straight forward.

One that I have tried is KMnO4 and SO2. Capture the SO2 in an erlenmeyer,
which will look empty. Remove the lid (it should be tightly sealed up until
the point that you are doing the demonstration!!) and pour in a "wine"
coloured solution of KMnO4. The colour instantly disappears. The patter is
that it is "wine into water", with the permanganate being the "wine". Of
course, it is simply an redox reaction with the permanganate ending up as 
a colourless Mn(IV) species.

On the whole, though, I would recommend the above books as a superior 
source of well tested chemical demonstrations. And they will certainly 
have something that you could try.