Don't be scared by the fact you are told you can do anything... This is actually a good sign that the school recognises different opinions on methodology, and the freedom means that you can try many methods to see which work for you.
My CELTA lesson plans sometimes took 8 hours to prepare (for just 40 minutes of teaching). However, you are no longer preparing lesson plans for assessment, although you may need to submit them to your DOS.
Remember the patterns of lesson staging that you learnt on your TEFL - PPP or ESA for example. To begin with, stick with these and pick activities for each section, then focus on how to link them together. For the activities focus on what the students will do and how you will give instructions (maybe even script them for lower levels - this should not take too long... if it does then your instructions are too complicated).
With time you will start to remember activities you used before and perhaps even have the materials ready to go, or at least know where to photocopy bits you need. This will reduce your lesson planning immensely.
Finally, it's a good idea to start with the lesson objective (At the end of the lesson students will be able to) - think of one point that you want students to take home, if they learn nothing else in the lesson. I like to think of this as the take-away value. Your plan should then drive at this point, sometimes from different angles. Plus if you know what this essential point is, you can adapt far easier in class if the plan goes wrong.