Your car should already have a valet key. The left the factory with one valet key and two fob keys.
The valet key will not work the glovebox, the inside trunk release lockout, the rear seat fold down lock, or the external trunk release. I have never had a use for any of those items anyway.
Just an FYI: if you have to normally lock and unlock your doors, you will be forced to do this manually with the valet key (obviously), and that means placing a lot more use on the microswitches for that purpose in the door latch. These are already somewhat fragile as it is, and you will be asking these components to do perhaps a much higher duty cycle than was perhaps intended.
I do not have any other keys on the key for my cars, I always have them on a separate keychain. I hate things dangling in the car.
I find in this way, the fob works great. It is chunky enough that I can easily feel it in my pocket, yet when folded it not going to something sinister like poke me in the scrote.