So when I go to the Power Options in Windows, there's an option to have your HDDs turned off during inactivity. What would happen if my HDD was turned off while I still had work opened/unsaved data? Would I lose it all? And are there any other good things/bad things of doing it?
Windows will flush files to the hard drive before it powers down. I personally don't think it makes much of a difference in power usage, and it may actually shorten the life of the hard drive due to power surges while the drive spins back up after being powered down, unless you are talking about laptop hard drives which are made to power down for increased battery life.
There's no downside to spinning the drives down after inactivity, save for the fact that you'll have to wait a few seconds for them to spin back up again when you resume work.
Modern hard drives use offloading ramps which means the heads never touch the surface of the disc - so they're not subject to the same head wear issues when stopping and stopping that older drives were. Drives these days are designed with the expectation that they can be put into a low-power "spun-down" mode. "Power surges" are not an issue - if they were then your drives would be just as vulnerable when you first turn your machine on in the morning and when you shut it down in the evening.
Unsaved data will still be in RAM as long as you don't turn the power off - when you return to the machine, even if only to shut it down, Windows will then spin up the drives and flush it's buffers to the disk before it shuts down.