You can do so much in the bath: read a book; listen to music; pretend to be a mermaid; or invent whole new card games for use in your upcoming RPG. At least, that's what The Witcher 3's senior gameplay designer Damien Monnier did with Gwent.
In an interview with Kotaku, Monnier says that he was given one weekend to come up with a new game for The Witcher 3 to fit where the dice poker game had fit in The Witcher 2. On Sunday evening he had a bath, and while he was in the tub the basic idea washed over him:
"You have a number that's higher than the other person's number, then you have things that will affect that number. Some cards double it, some cards lower it, and ultimately the challenge comes from the initial hand. So, it's about managing your initial hand and bluffing."
By the time he was done with his bath, he had the basic rules. He mocked up some cards and took them into work to play with the studio director Adam Badowski, and it was a hit.
The interview also has some more interesting tidbits about the game itself—apparently it was originally going to be three-player but it didn't look right in the game, and while the physical version is fair the digital version gives the player an unfair advantage—but the bath bit is definitely key here. Clearly baths are very important to game development.
Unfortunately we don't get any details as to the specific kind of bath Monnier had, but if you too want to be a successful game designer you should probably start experimenting with water temperatures and bath products tonight. As you can see, the witcher himself favours a freestanding wooden keg variety. Let me know when you hit on the formula for success.