Can I have extra free time to shower, dress, and generally hang about enjoying the warmth and ambience of the dungeon for nothing?

No. 

The Dungeon Keeper will expect to have access to his facilities at the end of your hire to prepare them for his or others use. 

If you wish to extend your hire you will need to have arranged this with the Dungeon Keeper at the beginning of the hire at the latest. Circumstances might make it not possible to extend your hire.

You will be expected to be finished and ready to leave at the end of the time you have paid for. Your time is for you to use as you wish... The Dungeon Keepers time is his time and he is not really interested in giving it away for free.

However he is a friendly chap and, if you have not trashed his pride and joy and left things in a mess (in which case he will definitely not be friendly), he will enjoy chatting with you to ensure everything was as you wished and that you had a jolly good time.You wont be rushed out the door and if you need to wait for a cab to arrive or just chat with him that is all fine.