This activity is designed for turning statements into questions, but I am sure you can easily adapt it to other grammar points as long as there is always only one possible answer. The worksheet attached was designed to help with page 20 of FROM - 英文法基礎ドリル.
How it works:
Give each student a worksheet and tell them to turn the statements into questions. Make sure to note that it's important to leave a blank space for every space that occurs in the sentence. Point out the little numbers that are found below some of the spaces in the answer sheet. These numbers correspond to the numbers found at the bottom of the worksheet. By filling out the worksheet students will find the letters that correlate to that number and they will then be able to find the hidden message.
*If planning to use the attached worksheet, the hidden message is "have a nice day".
Important note:
Students will sometimes try to guess the hidden message before completing the worksheet. So, it's important to tell them that they need to answer each question correctly AND find the hidden message to get full credit for it.
General note:
I gave "special stickers" (character-themed stickers I brought with me from the US) to give out to students as a prize to those who completed the worksheet and found the hidden message.
I made this game specifically for a class that has a lot of quiet students that don't usually participate in group activities. I found that they were receptive to this activity as it let them participate without having to worry about pairwork.