If your department allows additional carriers and accoutrements to the duty belt, just what is out there today? Probably two things have been added without question. One is the electronic control device or ECD (TASER comes to mind for my department). The other is a glove carrier. Nearly every officer I have met carries extra gloves for protection and it is the proper thing to do.
But what are other "things" that are out there? Cell phones seem to be very popular on the belt and have replaced pagers. What is important to remember is to put the cell on a silent or vibrate mode to maintain sound discipline. I'm sure you can imagine an officer on a building check and his cell goes off. Not good. Bottom line, put it on silent.
Knives and sometimes multi-tools are present, but some departments have a restriction on folding knives on the duty belt. I, for one, have a regulation that if you carry a knife on a duty it must be engraved with your name or number. I have carried a folding knife on my belt for my entire career. I know several officers who carry a Swiss Army knife with a variety of tools to consolidate. But I'll stop here. Knives are like handguns; the discussion can go on forever.
Keys are another thing. There is an old saying that you can tell how important you are by the number of keys you have on your key ring. I only have two suggestions here. Have a backup key in your pocket or somewhere else. Invariably, officers lose their keys and the problem of how to get the patrol car started is solved by a backup key.
Next, get yourself some rubber bands and wrap the keys for noise control. Your next building check, listen for the other cops in the building. You'll be able to hear the jangling of their keys in addition to loud radios and ringing cell phones. No need for anything special to quiet keys; just rubber bands.