I have a Chromebook I routinely use for things like e-banking and other things where a security breach would be catastrophic. I've read it is a very secure OS by nature due to its small surface area for attacks. That said, are there any steps I should take to harden my Chromebook? Or is it as secure as it can be right out of the box?
For things like e-banking Chrome devices are secure by default. You don't need to do anything special. There is a good overview about the security here, and a hardening guide here. But you should be aware that your Google account is in fact a part of the system so you should protect the account by things like using a strong password and two factor authentication to maximize security.