Please, for all that is e-readers, put a case on your Kindle. I can't tell you how many times I've dropped this thing while packing and re-packing, while on a crowded subway, while...reading it. I'm so glad that I had a case on this thing, if only for making sure the white face stayed white (even if you can't really see it).
Dropping a Kindle is not nearly as devastating as dropping a phone, but you can easily crack the screen and damage the pixels. Also, why wouldn't you want to spend an extra $20ish to protect something you invested much more than that in?
This was the Kindle case I bought immediately, mainly because it doesn't look like a Kindle case. It looks like a journal or old book, which is much more my speed. It's satisfying to hold and use (the magnet in the edge keeps the cover secure and turns the Kindle on and off) and it comes in multiple colors if red isn't your thing.
A padded, lightweight case is also a good option, complete with elastics on each corner to hold your Kindle in place. It's durable and easy to hold with one hand, as the case cover flips over and folds behind it.
You could just use a ziplock bag or one of those cheap plastic cases to keep your Kindle waterproof, but why not just get a waterproof case? If you plan to travel in the wet weather, or you're an avid bath-reader, a waterproof case is a worthwhile investment.
If you're looking for something a bit more utilitarian, why not look into this one from MoKo? It easily folds open for one-handed reading, but you can also flip it over twice to reveal an easy-to-use kickstand that can be adjusted to whichever height feels the most comfortable.