I usually avoid carrying too much stuff attached to my PFD because of the risk of entanglement. However, recently I started carrying a few more items, and I have to say I enjoy the extra piece of mind.
The gear I have been carrying in my PFD these days includes: whistle, signal mirror, small flares, cell phone in a water-proof case, knife, and strobe light. All this goes in small pockets and tabs/holders that came with my PFD.
I also keep a tow rope nearby, which sometimes goes attached to my torso. This is usually the largest piece of gear that I carry in my PFD.
I have not found a reasonably comfortable way to attach my VHF radio to the PFD yet, so I keep it handy in the cockpit and tethered to the kayak.