Re: Essential fly fishing gear/accessories.
No matter the type of fishing, I've gotten into the habit of carrying nippers, hemostats and/or long nose pliers, a multi tool Swiss Army knife and some other type of knife. I have several small pocket knives & several stationary blade knives. Depending on where I'm going & what I'm doing will dictate the knives I'll carry. I also always carry a sharpening stone or file for hooks & the knives.
Fred, that whistle is a great idea. I've read that before, but never obtained one.
Other items I feel are essentials are sunglasses, and a proper hat. Sunscreen or appropriate clothing should be essential depending on the conditions. Skin cancer is no joke!
IMO, spools of line (tippet or otherwise) & spare leaders are essential but should not be something you have to decide on or not.
If you fish the surf or big water from a boat or wading, a good stripping basket can be essential, but that would likely be situation dependent. They can get in the way too.
I'm the opposite with glasses. Mine have bi-focals built in, but frankly I can see great up close. My sight becomes an issue beyond about 3 ft.
Remember, no one likes to be behind the big truck, but that's better than being under it!