Backstory: Every hotel has a fob or a card for your room key. I've lost quite a few and it's a pain to get a new one when the staff is busy and the line is long. I almost never lose my phone, having a backup bluetooth app to open the hotel door would be awesome.
The Idea: A mobile hotel app that doubles as a room key.
Deep Dive: You would have to partner with a smaller hotel and install custom door locks that work with bluetooth devices. You could build additional features inside the app such as ordering room service and upgrading the tv package to get a hotel to fund the idea.
Validation: This already exists for personal home residence.
Monetization: This is the perfect opportunity for a hotel to capitalize on additional revenue through an app. Based on the additional revenue generated for the hotel, you would be able to charge a monthly fee or work out a revenue share from the sales inside the app.