There is very little you can write which will "stress" the CPU more than running, say, a game. Specificially, a game will also stress the GPU, hence probably suck your laptop battery more quickly.

I've seen another project recently to attempt to heat the CPU as much as possible - but that was written in asm, as it depends a great deal on exercising all parts of the CPU in the right order, to achieve maximum parallelisation and using things such as MMX instructions, and hard FP operations such as exponents, square roots, trig etc. It's highly CPU specific (I mean, down to the particular core version).

Oh yes, and you probably don't want to use goto in C, in most cases.