I am using a Macbook pro (Sierra 10.12.1), under a non-admin account and I do not have the admin password. I was wondering how much of my data (passwords, files, etc) is accessible by the admin?
A user with administrator privileges can read all your unencrypted files. And you have to assume the access to your data can happen without you knowing it (an administrator can simply take backup of the system and access the backed up files offline leaving no trace on the system).
One exception is the keychain file (the one containing all your saved passwords). It is encrypted and, by default, protected with the same password as the one you use for login. An administrator cannot access the contents and cannot change the password to this file without knowing the current one. Thus the administrator cannot access your saved passwords, credentials, and other data which you store in the Keychain etc.
The above applies only to passive "attack". A rogue administrator can install a key logger and simply learn your password by sniffing
The admin can't simply open up your files in finder and call it good. If the admin is using a finder window, they would have to actually change the file permissions to allow read and write to all users, under "get info". They can also change your password and then log in as you.
In terminal, the admin can access all the files as well as change your password with sudo.