Normally I have a very simple sedentary lifestyle, but

Try wearing a pedometer during your regular and non-regular days. Log whatever extra steps you get on the non-regular days. Or, you could calculate what your calorie goal would be if you were active and then just add an exercise for the extra calories. Or, if this is an ongoing thing, I would guess you'd be fine to put yourself as lightly active and just eat that amount daily. It sounds like you are more than lightly active almost 1/2 the week and sedentary is considered under 5000 steps per day so you may not even be that sedentary on a normal day.

Thanks! I was hoping there was some magic way to do it lol. I really need a pedometer (or a FitBit..)

You definitely need a FitBit. On a lazy day, I barely burn 1400. On a busy day at work it's in the 1800's. On days I work out - well, you get the idea.