[jonathanschaaij]

At night before bed you have the chance to go to the toilet. Just before sleeping you have to take a soda bottle and put the keys of all bathrooms inside. Now fill the bottle with water and put it in the freezer. Then drink at least two glasses of water. Now you can go to sleep. The next morning you can take the bottle out of the freezer, but you can't heat it up. Every hour you've got to drink everything that has melted. once the bottle in empty the keys can fall out and you can unlock the bathroom and release yourself.
You can make it harder for yourself by drinking more water before sleeping or by using a larger soda bottle. If you think this still isn't hard enough you can put the bathroom keys in a lockable container and the keys to the container in an other bottle. this way you would need to wait for too containers to melt and also drink twice as much.