Ever played the drinking game "21"?
Sit in a circle, and take turns going round the circle counting up to 21 with each person saying one, two or three consecutive numbers until the count reaches 21 and the one who says that has to do a forfeit (usually down their drink).
To make it interesting, the rules mean that if you say two numbers, then the direction round the circle changes, effectively passing the buck to the person who went before you. Say three numbers and it skips the next person in the line and the next person picks up.
Once you've got to 21, the person who did the forfeit can choose to replace any number with a word or phrase. Then you start again but perhaps the number four is now "John smells of cheese". e.g. "1", "2","3","John smells of cheese", "5", "6" etc. (you can try and get John to say 4 for added amusement")
We soon got tired of this because people just got too good at it. So, to make it harder you can play using roman numerals.
So counting up, you'd say "I", "II", "III", "IV", "V", "VI" etc all the way up to "XXI"
Then, to make it more interesting, you can replace I, V, and X with other words. We used sweary words cause we're childish. But if you wanted a clean version, you could change "I" to "Ship", "V" to "Tank" and "X" to "Bullocks"
then sit in a circle chanting:
"Ship", "ShipShip", "ShipShipShip", "ShipTank", "Tank", "TankShip", all the way up to "BullocksBullocksShip"
It certainly give the brain a bit of a work out!