If you need to extend a visit, or in case you’re facing a re-routing on a business trip, then you should select your hotel with care. The most expensive way to get a room is to walk in the front door and buy one at the desk. Even the hotel you’re already staying at might be the second best option.
Many of the hotels sell their remaining rooms of the day via internet booking agencies at low rates. Try places like HRS.com or Hotels.com in the afternoon, well before your cancellation time, to check for interesting hotels. Even if you should get a small room, after all, you’ll be in the Grand Hotel, and you’ll be using the spa!
Check your terms & conditions of the hotel you’re checked into, or the reservation you’re about to use. Many booking servers allow you to cancel on arrival day before 18:00. So if you get Grand Hotel at 17:00 for just a few €€€… lucky you!