There are multiple passwords in the TC..
You can setup a main network and a guest network.
Guest network has no access to the LAN including the TC hard disk.. only internet.
There are limitations..
1. The TC must be the main router of the network.
2. All security on the TC is a 1sec press of the reset away.. whereupon everything returns to default passwords so anyone can fix the passwords to what they like.. or a 10sec press for full reset.
In other words. Physical security of the TC is absolutely essential to this.
3. There is no control over internet usage by the renters.. if you want to actually limit them the TC only offers time limits.. a better router would be anything that handles gargoyle.. just look it up.. and it will then allow you full network control and you can set quotas.