For the most part, the answer is yes, you can waste money on your Prius like you can for any other car.
I would just caution you to avoid anything that might clog fuel injectors. Also, the exhaust monitoring in the Prius is more picky than other cars, so anything that creates funny burn by-products might set off the check engine light.
And no more than 15% ethanol.
Have fun. Let us know what improvements you measure (and how you control the experiment to avoid other factors from overwhelming the measurement).