Clients that won't pay to get a dead squirrel out of the lower chimney "Don't worry, he will rot pretty soon", and the guests have to leave because the place reeks of dead animal, and we have to refund them a week's stay. 

Squirrels love attics.

We purchased the property in August, did the clean out ourselves (previous tenant left a mess- clothes, mold, stole equipment and appliances, left a car, oh and had dogs, cats, and squirrels living in the house!)

I have cut down and cut back trees proactively,  Mostly dead trees or black walnuts which attract squirrels. 

I don't even ask our tenants to change the filters since sometimes getting the metal pieces of the furnace back together takes a little mechanical talent and when the filter is inside the furnace there are a few bad things that occur e.g. if the door is not properly closed the safety switch will be open and the furnace will not operate and you get to pay a technician to solve the problem and tenants sometimes don't installs filters correctly and the filters get sucked up into the blower squirrel cage.

I gravitate towards the side of passive investor, but I also understand that this detour could be an unplanned opportunity.

I refer to it as .....squirrel......or a more popular phrase is shiny object. 

You could sell them for some money, but it isn't worth the hassle and loss of goodwill.This may feel like a big deal as a new landlord, but it is par for the course and there should be a repair and replacement fund to handle such unplanned expenses. 

For things like squirrels that get in through access points we generally cover the access points ad removal. 

Make sure you budget (time and money) for some unknowns as you will definitely run into a few unplanned thing on these old houses.as others have mentioned, you can update electric, insulation, etc without removing the plaster if you want.