CalvinBall is a game with only one rule, no rule can be used more than once. The game was originally created by Calvin in defiance of organised sport after he was ridiculed at school while playing baseball. All players must wear masks though as to why Calvin will only say "no one is allowed to question the masks". The game uses various tools including flags, wickets and a ball (usually a soccer ball but any will do). Scoring appears to be completely random, with anything from 'Q' to '12' to 'oogy' to 'boogy' having been registered. The only person to have played the game apart from Calvin and Hobbes is Rosalyn who seemed to pick up the rules (or lack thereof) very quickly and win a game.
G.R.O.S.S. or Get Rid Of Slimy girlS is Calvin and Hobbes anti-girl club. It's main purpose is to discuss how horrible girls are and to come up with plans to 'get them'. The base for G.R.O.S.S. is in Calvin's treehouse and requires wearing of a newspaper hat and singing of the G.R.O.S.S. song to enter. Calvin and Hobbes spend a lot of time bickering over hierarchy in the club and often end up fighting over who's higher up. The only time this seems to end is when Susie Derkins appears, then all hostilities are focused on her. Wether they manage to get Susie or not, Calvin and Hobbes both get awarded with medals for their work and bravery in G.R.O.S.S.
Calvin and Hobbes often go time travelling with the aid of a versatile cardboard box (also used for transmogrification and duplication). A pair of goggles is donned before entering the box as a safety measure in order to contend with vortices and light speeds. Then, if you want to go forward in time, you sit forward in the box and if you want to go backwards in time, you face the back. This was discovered when, intending to go into the future to bring back some futuristic inventions, Calvin and Hobbes accidentally ended up in the Cretaceous period face to face with a gigantic dinosaur.
The transmogrifier is merely the cardboard box used for time travel turned upside down. An arrow is then placed on the side of the box and several options written down. All you then have to do is point the arrow at what you want to be, get in the transmogrifier and 'Zap' you become whatever it is. Calvin first tried this machine to become a tiger but has been several different things including a safe and an elephant. Calvin has now placed transmogrification technology inside a water pistol he owns to make a portable version of the machine. This came in very handy for him when he found himself falling from a great height.
The duplicator is created by placing the cardboard box on its side and writing 'duplicator' on it. Anything that goes into the box then goes 'boink' and multiplies. Calvin decides to make duplicates of himself to go to school for him and to do all his household chores but because they are all Calvins, they don't want to do the jobs either so he turns them all into worms with his transmogrifier. Because of this, Calvin adds an 'Ethicator' switch on to the duplicator and creates a good version of himself which starts off very well but as soon as it has a bad thought (which considering it's a Calvin didn't take very long) it disappeared.
During the winter months when it snows, Calvin likes to show off his artistic side by building snowmen and snow sculptures. Being the kind of child he is though, Calvin doesn't just build ordinary snowmen. He builds them with deformities like several heads, he builds them with heads fallen off, he builds them buried up to the neck and with trees groing straight through them. He also uses this time for his other winter hobbies, sledging and snowball fights. During both of these activities he always seems to end up face down in the snow and complaining to Hobbes.