Months could be lost just playing with the packaged instruments that are in NI Reaktor, let alone getting under the hood and building your own instruments.

My personal favourite so far is Metaphysical Function which has 12 noise/tone generators, a sample playback module, and a brilliant effects chain to run everything through for sound mangling. Automation can then be recorded to most controls to turn it into an undulating, pulsing monstrosity!