Dear Moon Unit,
Raymond Murphy’s grammar books are all excellent. They were written for students and I think that teachers sometimes need more in-depth explanation. I suggest you look at books by Michael Swan for this.
Here are the general rules concerning the use of at, in and on with time expression:
On is used with a day or date; e.g. on Monday, on 31st August.
At is used with a period of time; e.g. at Easter at Christmas at the weekend. At is also used with time; e.g. at 2 o’clock.
In is used with a section of the day: in the morning, in the evening, in the afternoon but note that we say: at night. In is also used with months (in February), decades (in the 1980’s), years (in 1997) and seasons (in winter).
Phrases like Monday night and Tuesday morning follow the rules for days and so take “on”.
I would treat “at the same time” and “at the moment” separately from the above to avoid confusion or overload. I advise you to leave out the exceptions in Murphy until your students have mastered the basics.
As follow up activities, you could take the same sentences as last week and work on them differently. Examples for this are: filling in blanks, students testing each other, re-organising jumbled sentences. You could even take one of the exercises and just do it again to see whether students perform better this time around.
I then suggest you follow up with a listening and speaking activity. Talking about diaries and appointments is a classic way to practise the use of these prepositions. Try to find a recording of two people trying to meet and discussing their respective availabilities. Do some work on general understanding and then focus on the language used in the recording. You could also make a recording yourself with a colleague.
Give students a copy of blank diary pages covering two weeks. Ask each of them to fill in certain activities on various days over those two weeks. Tell them not to leave too many blank spaces. Then tell them they have to meet each other at least four times over the coming two weeks and they talk about when this is possible.
Obviously the above will also require knowledge of future tenses; if your students are not ready for this, work on past activities using a diary. You could, of course, do both future and past work but I would suggest doing this in different lessons.