K bikes have been around for a long time, and are an extremely great piece of engineering. However a lot of owners have added options and solutions to perceived problems ( real or imaginary). Over the 20 years of ownership I have added quite a few.  What were your best  modifications and what were your worst.
Regards Martin.

The best? The nut on top.
The worst? Two bloody great spotlights sticking out either side of an RS fairing.

Comfort seat.

Cruise control.

A leggy blond as pillion.

Most significant was an Aeroflow headlight protector.  The one on my RT was shattered by a stone about 2000 miles from home, but the headlight lens was unscathed and made the rest of the trip.

Russel Daylong seat made Iron Butt runs a piece of cake.

10Watt LED spotlights for daytime visibility

Fuel cooler in front of the radiator made a significant reduction in tank temperature.

Run-n-Lights made a big difference in night time and brake visibility.

Added large ScotchLite reflectoor stickers to panniers for added visibility to the rear at night.

Eastern Beaver headlight relay harness to boost headlight brightness. 

K1100 centerstands added to my 2valve bikes to replace the original centerstand which can rust out.

Being an American, I converted the clock to 12 hour from 24 hour.

Added safety straps to secure the city cases to the bike after losing two cases. 

Attached my EZ Pass transponder to the inside of the rear cowl cover.

Spiegler stainless steel brake lines.

Replaced the original Y spoke wheels with three spoke which are infinitely easier to clean.

Till's Fuel Injectors
https://www.tills.de/bmw-k75/fuel-system-205/

the best thing I've ever bought
at my first ride after the upgrade I couldn't help feeling the smoothness of an inline six ICE

On my OZ-based '87 K100RS, now in Charlie's hands, adding K1100RS mirror spacers, extending the 'bars outward each end with internal friction 50mm extenders, 25mm risers, and Suburban Machinery's Throttlemeister cruise/end weights, added up to wonderful comfort over the road - less forward lean, and more control due to handlebar width.

The Laminar Lip, added later, gave a bit of extra comfort from wind swirl at open road speeds, for my height, too. And believe it or not, the hard as a rock Corbin leather bucket seat, which came with the bike, had been one of the most comfortable all day perches, for a BMW, I have ridden on. The best being the stock, plushy heated seat of my long-gone K1200GT Brick.

+1 on the Corbin Rock seat. I went through a heap of different seat iterations with only slight improvements. I managed to pick up a mixed bag of K bits of Ebay, a Haynes workshop manual, a set of fabric panniers, a new after market rear shock and a Corbin seat for $100 Au. I was after the shock which proved to be nearly rigid and which I later sold for $150.00. Due to the hardness of the Corbin I was going to pass it on. However I decided to give it a go, and was amazed how something so hard can be so comfortable.

The K Cable - makes it so easy to add and remove electrical accessories

Tills - THE K-Cable