Sing songs :)

For gr3/4, you can find good songs on this site (or simply search youtube directly for songs that deal with the language of the lesson). Singing is a good way to get the attention of the students at the start of the lesson, and it uses up a few minutes so that the textbook activities for periods 1/2 will take up the remaining time. And students don't mind repetition of songs - one of my coteachers sings "Lemon Tree" twice at the start of every gr3 lesson. She's been doing this for about two months and they're still happy doing it.

Don't be afraid of the fact that they are all the same words. These kids know hardly any English and need to keep hearing and practicing the words they know (imagine you were taking a class in Korean; I'm guessing you'd feel the same).

For the very short lessons (esp period 3), you can download extra worksheets etc from the book's website (tsherpa.co.kr), although you'll need an ID. That said, the activities you can find on this site are generally very popular at my schools. I usually play a ppt game with them (pass the ball/4 corners etc) when the textbook doesn't have enough to keep them busy.