Is there an application / ROM that allows me to set the phone to "Guest" mode which I can configure?
You might want to check for "app lockers" like e.g. App Lock, which allow you to password-protect apps, or use even more restrictive child protection apps like e.g. Child App Protector, which can sandbox an environment (also called "Kiosk mode").

A somehow "special" solution would be App Locker II: Fake Crash, which in "protected mode" displays a fake force-close message so people do not get the idea you protected something.