1) Finish the lesson with Sight Reading
There are lots of Sight Reading books out there. Sight Reading pieces are short so you can do as many or as few as you like - right up to the end of the lesson without running over. Check out these sight reading cards for Pre-Primer/Primer students
Primer Sight Reader cards
Pre-Primer Sight Reading Cards
2) Finish the lesson with a theory game
Some theory games take longer than others. Select a theory game that will take a little less than the remaining lesson time (you need to leave a minute or two to celebrate the winner and talk about what they learned through the game).
3) Finish the lesson with a group piece already completed - a mini concert
It's good to learn new music - after all, that's why the students come to us, but it's really nice for the them to play something they already know, just to enjoy the music and the experience. Group pieces played well are particularly satisfying.
4) Finish the lesson with a chat
One time, one of my student forgot his music book at home. While I was searching my library to see if I had another copy for him to use, he said "Miss Julia, shall we just sit and talk instead? Can I share something with the class?"
I told him that his mom wasn't paying for him to come and talk, but I did allow him to share what turned out to be a rather long story about his iPad, while I looked for a book for him.
Though I had to cut his story short (I didn't want the lesson to become a group discussion on iPad games), it reminded me of the importance of getting to know the students. Not just talking "piano" in the lesson, but allowing time to find out about them - what they like, what they dislike, their hobbies, their favorites etc. These brief chats help the students and the teachers to get to know each other.