Ask HN: What are some ideas for how to best use idle compute time?

I have a small lab at home that I put together to learn and develop Kubernetes skills. While this is still the primary use, the compute time is mostly idle. What are some ideas for how best to use this idle compute time?

Even better if it results in some passive income. One idea I had is to use the 4 nodes to mine bitcoin. Supposedly, you can use CPU as well as GPU to mine but I'm still investigating.