Yes there is book with 5 minute activities. Actually its titel is "Five-Minute Activities" . A recource book of short acrivities . Publisher: Cambridge University Press. ISBN: 9780521397810
I work with it and it helps me a lot. It can be used for diffrent levels. I paid 22,50 € but you can have look on ebay!
I have the one for adults but there is another one for young learners, which don't know.
I never explain why a letter is pronounced in diffrent ways! Tell them it is like it is!
Have a look at:www.cambridge.org