It's true that students come into the classroom "cool" and need some warming up before they are ready to tackle the serious business of learning English. This warm-up period helps them get ready for thinking in English. So my advice is whatever you choose as a warm-up activity, it should be at a level that is easy for them and it should be quick (ie no more than 5 minutes or 10 minutes max).
The warmer can be in a lead-in to the main aim of the class, it can be a review of previous lessons or it can be totally unrelated.
You could play hangman and use a word that is related to what you are about to study. When the students have guessed the word they can brainstorm vocab around the topic and / or discuss what they know about it.
You could play noughts and crosses. Each square on the grid has a subject, eg past tenses, pronunciation, spelling. Split the class into two groups, one is noughts and the other crosses. One of the groups chooses a square and you ask the corresponding question. If they get the answer right, they can place their nought or cross in the square and the turn passes to the other group. As in the traditional game, the first group to get a line of noughts or crosses wins. You can adapt this to review work from the previous week.
You can use a colourful picture with lots of detail that you hold facing you. Get everybody's attention and flash the picture at them for a split second. They call out anything they have seen. At first, this will only be colours. You can then flash the picture again, they tell you what they have seen. Continue until you feel they have a fair idea of the picture and finally show it to them. The picture could be related to the topic of the lesson. If so, students can then brainstorm related vocab and / or tell each other in pairs what they know about the topic.
If you have access to an overhead projector, you can draw strange shapes on a transparency and project them onto the wall. Students talk about what they might be. You could also show them a picture that you reveal gradually, while they try to guess what it is.
Students could simply write down anything they remember from the previous lesson. This could take the form of an e-mail or postcard to a friend. They pretend they are writing to someone to describe what they have been doing.
Students could simply in groups or pairs talk about what they have done since the last time they saw each other, in English of course. This would be an authentic and natural activity.
One final point, it's a good idea to go easy on yourself and keep the preparation of warmers to a minimum. You can find that some warmers take as much preparation as a full-blown activity.