Depending upon the type of class you are teaching, the following worked successfully for me.
5-10 mins. for "warm-up" = the mechanics, roll taking, distribution or collection of anything, announcements
10-15 mins for the activity of the week. One week we wrote for 5 mins (or more or less depending upon the level) in a journal and then we shared questions and answers, often voting on whether we were for or against a topic. Each class was different and they loved comparing how they voted. The next week we did activities, to get the students out of their chairs. Because I was teaching foreign language, this was always in the target language.
The "core lesson" was broken down into 10-15-20 mins, depending upon the attention-span of a particular class. This was always different. However, the students knew that the "core lesson" MUST be done each day, whether I was there or not, in order to be ready for the national/international exams they had in May.
At the end of class, be sure to leave time for any questions and/or beginning the homework so no one left the room without knowing what was to be done and why.
The greatest compliment for me was hearing "we are never bored in this class!"