One of the best ways to use liquid nitrogen is to make ice cream. If you have ready access to liquid nitrogen (you're in college, perhaps) then making ice cream is almost easier than buying it, and much more enjoyable.

Here is one recipe:

You'll also need a large plastic bowl (tupperware works well too) and a wooden spoon.

Mix the liquid ingredients together, then add the sugar and stir until it dissolves. Add the flavorings.

Now, add the liquid nitrogen in small amounts (approximately 250 ml at a time) and stir until the nitrogen has boiled completely away. Repeat until the ice cream is frozen.

I've made ice cream this way twice, and I have some additional advice. If you use chocolate, make sure that it is grated into very small pieces, because large chunks will be frozen too hard to eat. If you make butter pecan ice cream, drain the excess butter after sauteing (or stir very well), because otherwise it will separate into hard chunks of frozen greasy salty butter.