I have had a few interesting experiments here in the past year.
Experiment 0: I bought a hotel affiliate site off of flippa. It did pretty well initially, but I didn't do enough investigation into how the previous owner had been generating traffic. In short, there was a lot of untoward stuff going on. As I was getting all of that straightened out, the site got (deservedly) banned from Google's index for a few key terms. I made my money back and learned a valuable lesson: don't buy sites off of flippa.
Experiment 1: I created a few different sites around a big product launch, and monetized via product reviews and the Amazon Affiliate program. This worked very well for a period of time; the site was grossing $100 a day for several weeks with essentially 0 work. Slowly, my site dipped in the rankings for the key terms as much bigger players got their act together. From this, I learned that one-off sites can be valuable, but probably not in the long term. I should've sold the sites at their peak.
Experiment 2: I wrote some algorithms to find underpriced stocks and then examinate a few strategies around that security's options. This was actually a lot of fun. Based on my program, I ended up buying out-of-the-money puts on 5 or 6 different stocks. I'm sitting on a small profit right now. The next step is to exit my positions, finetune the algorithms based on a few key things I learned, and put more money into action.