I've taken keys to the Kingdom twice over the years and what you visit in the Utilidoors has changed dramatically over time. Currently you go down below Main Street USA and see some areas around there. You can view the tunnel toward Crystal palace including the AVAC tube for trash transport and depending on the guide you might see the one that heads toward Cinderella Castle.
When I took it the first time back in the 90's, you went down below Fantasyland and saw much, much more interesting areas. We came down a set of stairs right next to the DACS room you've likely seen on the behind the scenes TV shows. We went into the old costuming department and lockers (which I believe are moved to north service area now), saw the entrance to the Utilidoors behind Pinocchio Village House and viewed tunnel routes heading toward Tomorrowland, Cinderella Castle and Liberty Square. They also pointed out the fact that the only non-electric vehicle allowed in the Utilidoors was the armored car and they showed us the enclosed 'chute' it backed into on it's scheduled visits. At that time there was a cast member cafeteria right there near the entrance below Pinocchio's with costuming across the hall. While today isn't exactly boring for the big Disney Parks fan, they used to make much, much more interesting areas accessible for you to see.