How to prepare lessons as a visiting professor?
This is ultimately a very broad question, but I will give you some starting points.


  Since I work full time in industry before I leave to teach, I am rather limited in preparing my lectures. So I have to be very efficient during preparation.


You are doing this pro bono, so I am assuming that you also want to do a good job teaching. Unfortunately, there is an obvious tradeoff between "being efficient" and teaching well - the material that you can prepare quickly is not necessarily the one that will work best didactically, especially if you start factoring in the "ratio of them being occupied versus you preparing" (a pretty bad metric to go by when it comes to prepare a good course, especially since it may lead to you assigning some sort of "busy work" to students to make the lecture time go by). Hence, my first suggestion is that you try to shift your focus away from trying to be efficient about your preparation, and more towards how to teach your subject in the most effective way. This may also mean that you need to prepare a bit in the evenings prior to leaving (or in the evenings while you are there). 


  What recommendations to you have to quickly prepare good lecture material (slides) for the lecture?


I feel the best way to do this, both in terms of efficiency and quality, is to find a good set of teaching materials (this can include book(s), slide sets, papers, or even online material in form of blog posts or video tutorials - there will be something useful out there for pretty much any topic) and reuse it. Many lecturers, myself included, have a fatal tendency to always want to do everything anew from scratch, because clearly no prebaked material will cover exactly what you want to teach in exactly the way you would teach it. I consider this a fallacy, especially if preparation time is of the essence. Do not be afraid to use all resources that are at your disposal, and enrich them with your industrial experience. You get no teaching brownie points for creating all your material from scratch. That being said, consider that collecting, screening, compiling, and annotating this material will take considerable time as well, so start early.


  What can be pitfalls I should avoid in preparation?


Depending on your background and personality, there are many potential pitfalls. Teaching is after all an activity that you get better in with experience. Generally speaking, and based on the pieces of info you provide, you should at least watch out for the following.


Your material needs to be suitable to the audience. Find out early the level of prior knowledge of your students, and adjust your material accordingly.
Make sure that the amount of material you prepare is in line with the length of the course. Don't try to shove 10 years of industrial experience into a two-week course.
Make sure that your course is not just a string of related industry talks that you have given previously. Especially if you have given talks before and time for preparation is short, there is a temptation to just re-give talks that you had prepared for other occasions. This will almost certainly not result in a compelling lecture.



  I was told (though I am not sure whether this is going to be true) that the students will have computers at their disposal. Would you prepare exercises? 


This depends on your topic, but for most topics I would say you should definitely prepare exercises. Not because exercises are more efficient in terms of preparation time (although they may be), but because, for many subjects, a good mix of lectures and exercises work much better in terms of retention than just lectures alone.

However, the fact that you don't even know whether there will be computers or not suggests to me that you will need to find out a bit more reliable information about the university where you will teach, its students, and the environment. It is hard to prepare a good course when you don't even have this basic information at the ready.