You'll need stop watches for this activity. Its best to have one per group.
Before making groups explain the language target with the JTE and tell the students they'll be doing three group challenges. I generally demonstrate the first challenge before handing out the worksheets and making groups. I find they listen better before making groups.
First Challenge: The students must write the word 'beautiful' nine times fast. As soon as they finish writing they put their pencils down. At the bottom of the worksheet they'll write the answer to the question 'Who is the fastest writer?' with answers such as 'I am the fastest writer' or '~ is the fastest writer'. Afterwards I sometimes ask a couple of students to report who is the fastest in there group.
Second Challenge: The students must speak the tongue twister 'A big black bug bit a big black bear' five times fast. I demonstrate this with the JTE taking turns doing the tongue twister and recording the time with the stop watch. After we demonstrate we practice a little with the students, make sure they know the meaning of the sentence and give them a little practice time. We then hand out the stop watches to each group and they take turns doing the tongue twister. As above the write who the fastest is at the bottom of the page.
The Third Challenge: After reading the question and checking the meaning 'Whose picture is the nicest?' I tell the students they have one minute or so to draw the ALT's face. After they draw it tell them they will walk around the classroom and check each group's pictures. They pick the one they like best and draw a star in the small box at the side. After everyone has returned to their seats they count stars to see the winner in their group. Here they write the answer 'My picture is the nicest' or '~'s picture is the nicest'.
Not a huge amount of practice for the target language here but it is fun.