There are a lot of activities and available on BusyTeacher if you are looking for ideas for a warmer or filler activity. In fact there are currently 360 free ice-breakers and warmers to choose from. The warm-up activity described here, for example, requires very little preparation and has been successfully used by other busy teachers. It would also be enjoyable for students, especially intermediate learners, who need some practice with questions and answers.
Having short 5-minute activities on hand is always important.
They can be used as warm ups to get students to start thinking about material you plan to use in your lesson and are an excellent opportunity to get students out of their seats, moving around, and speaking. These short exercises are also useful when you finish the lesson material earlier than expected. Perhaps your students were very eager to complete the lesson activities, performed better than expected, or, on the other hand, struggled or gave up. Whatever the reason, it is important for you to use this extra time well so that students get the most from their class time.