There are many activities. Given the time limitations For a pre-int tesol/celta class I'd suggest the following. It's easy to prepare and setup.

You can give or have each student draw a simple 7 day diary/calendar (just two columns day mon-sun and activity), it might be a good idea to include the real dates for next week to emphasis the future aspect. Tell ss that they can write 3 activities to fill 3 of the days. They can choose any activity and write it next to any day they wish on their calendar. The idea is that they fill the remaining four days with activities they want to do by asking others what they are going to do on the days they still have free and then arranging to do that activity with that person. Of course they cant double book their time so it requires students to ask multiple partners in order to fill all 7 days. At its most basic that's it!

However you can expand it by doing things like introducing language for making excuses. Let's say for example a student asks another student what they are going to do on Monday and that students replies he is going to go bungee jumping. If the student is free on Monday but they don't like the idea of going bungee jumping then they can make an excuse e.g. I'm going to wash my hair, I'm going to do my homework, etc. You elicit the types of excuses people commonly make prior to the activity.

Follow up activities could include getting students to go into more detail about the arrangements they've made e.g. where they are going to meet and at what time? How they are going to get there? What they are going to need to take?, etc.