Not the sexiest title to end the year on but probably an appropriate one given the circumstances.
The topic is about controlling critters, vectors (carriers of diseases - example mosquitoes) and things that have no business in your machines. This applies to other coin operated machinery as well. Mostly common sense but it was something I didn’t think about until I was redoing my garage and turning it into a game room. We have had tree rat problems here in California as well as mice. You may have similar issues where you live. When i carried out some boxes of old comic books (wearing gloves and PPEs because of the droppings I saw in the garage), a black rat jumped out of the box and ran away. The inside of the box had the comic books tampered down and chewed down for a nice nest.
For the first thing when I redid the garage was to seal the vents that had been chewed open with new grated vents . Initially I placed some rat traps by them and after seeing no takers, presumed the garage was critter free.
I tied Bounce sheets (fabric softener sheets) to the pinball machine insulated plug connections just in case since mice and rats apparently do not like the smell.
I have experimented with peppermint concentrate and that works too.
Downside to these is that you need to replenish the Bounce or peppermint concentrate but they tend to last and if you aren’t allergic they are not bad smells - though the peppermint is strong.
For those members who have arcades in their attics or basements, repair holes in walls as well because when things get cold, critters find places to get warm in.
Any leaky pipes or drips should be taken care of to prevent pooled water. Problem if hot in warm weather and moist for mosquitoes and not a good idea to have water on the ground with electrical connections.
Any other suggestions are welcome and anything to keep non-electrical things out of our machines. I thought of using cat urine as some sites have said but I would save that if I get a Mousin’ Around machine.
Plus family less inclined to play pinball with cat pee smell - though a great way to ensure quiet time? 
Have a safe and happy New Year everyone.
Another good post to read: How to clean pinball machines and not get sick