I'll start with a simple one.
But, this is indicative of the person I am and the mods I like to do.
When I bought the car, the seller had not been driving the car in the previous months. And his new Interstate battery failed to the point it could not be charged. So he warrantied it, and I got a car with a brand-new 9/15 dated Interstate battery. Kickass. I love Interstates.
So to prevent that from happening again, I added a CTEK battery charger which I had been using for one of the motorcycles today. When you own several vehicles and if they sit unused for any period of time, it makes sense to use chargers. I have a CTEK on every one of our vehicles except for my wife's GMC truck. It gets used constantly and is basically an AK-47 anyway.
So, here is the mod. The CTEK comes with a quick disconnect pigtail. You wire it to a battery connection and a ground. For this car, I opened the positive battery terminal cover and found one of the M10 terminals. Picked up hot battery connection there. I used a ground stud on the front strut tower for ground. The detail is in the routing of the wires. I found an unused port in the grommet to the air pump compartment and routed the ground wire through there, as if BMW designed it that way. See picture. When not in use, the pigtail is fully contained in the air pump compartment and it even stows away nicely.
Sure, you can hook up a battery charger to these cars in 5 minutes. Or, you can take 10 minutes and do it a little more carefully and get a BMW-like result. So that's what I did. Note that I have not cleaned the engine compartment - or anything else yet - on this car!