The problem with non-planners--which I'm sure you're already aware of--is that their last-minute decisions might involve you when you don't want to be involved, or they might not have the initiative to actually do anything, so that even though "DH is spending time with his mom," they're actually just sitting around your living room, and you're not able to get anything done on your own.
So, you might try being a bit proactive, just out of self-preservation. Like, sit down with DH and go through each day that MIL will be there (or might be there) and decide who "gets" her for each block of free time. Like, "Okay, I'm going to run some errands on Wednesday afternoon, and she can come with me on those if she wants. But, Thursday afternoon I need to clean, so you need to take her out of the house. I don't care what you guys do, but you need to be out of the house from 1pm to 5pm, and feed yourselves dinner." And then keep talking about it a lot, like, "MIL, you must be looking forward to Thursday afternoon, getting to spend some time with DH out and about!" So that it just becomes natural and expected for them to leave the house together that afternoon, and you can get stuff done. That way you're not really "planning stuff" for them, but you're setting up expectations that will help you.
Kind of like you would, I presume, with meal schedules, sleep arrangements, etc.. Just think of it like that--you're all probably assuming MIL will sleep on the fold-out couch (or whatever), right? That's not really a formal "plan," but you would be really surprised if she came over and tried to take over your bed, or expected you to find room for a huge air mattress, or something. So maybe something like, "You'll be on the fold-out couch in the den, we eat dinner at 6pm, on Saturday morning we're all going to the farmers' market together, Tuesday and Thursday you'll get to hang out with DH while I'm running errands, here's where we keep the coffee..."