Is it possible to simulate use over some days?

There's probably fancy official software doing that, but I imagine you could pull off the same just by using Handbrake (free) to convert a large batch of videos from one format to another. That should work all cores of the CPU and the drives.

To work the GPU you could start a video and leave it on a loop setting or with a large queue of videos to play.

You can run Memtest or Rember in a loop to work the memory.

I've never done all three at the same time, but I see no reason why it wouldn't work.

Check out Sikuli to do some simple automation. I've used it for a few things I wanted to do quickly. Shouldn't be hard to do things like move files back and forth, keep pressing play on a video, etc.

There are thousands of ways to do this. One of the simplest is to create a workflow using the Automator app, which comes with OS X, and then run it. If you have lots of images you could create an automator workflow to add thumbnails or resize lots of images and then run that workflow multiple times. If you have the Pixelmator app, then you can use the Watermark Images action. Effectively you'll be performing a batch action over and over again.

This particular action is the most stressful that I've used and will stress the CPU, memory and hard drive.