It sounds as if this school is asking you to do quite a lot! I agree with what you say, you can just encourage employees to speak to you in English. It's important to correct their errors. Speaking with you will give them more confidence and the error correction will help them to learn. I suggest you don't spend any time planning for this, just go with whatever the school employees are doing and what they want to know about. You might even enjoy it and make some good friends there.
As for a new method, I don't know what methods you are using at the moment. You could try the task-based approach. The following is a simplified explanation of what this approach is. Give students an oral task to do which is slightly higher than what they are used to; but not so much higher that it is extremely challenging. You can choose any speaking activity from their coursebook or elsewhere. While they are working, note down the errors that they make, particularly in the target language of the exercise. For example if the activity is fixing a date to see a friend, note down errors in the present continuous (what are you doing on Saturday?) and prepositions of time (on Monday, at 10.00, in the afternoon). You then revise the language or give a full presentation of the structure. Whether this is brief revision of the language or a full presentation of the language items will depend on how well or how badly the students performed in the first exercise. You can then follow up this presentation with another exercise to practise the language. It can be fun to do a recording of the two exercises so students can compare how well they did before and after the presentation of the language points.
If you would like to try a different method, please write in again saying which methods you are using now.