On the thermometer, I used a DozyAnt from Amazon. I have used their stuff before, and it is pretty accurate. I don't mind poking holes in the lid for something I am going to actually use. Now if this was a 1970s kettle different story, but even then, it's your grill, do what you want...
WHat I consider basic accessories are...
#1. Tongs. Stainless steel, that can lock shut and hang.
#2. Aluminum catch pans. The aluminum lasagne pans are fine, but the Weber branded catch pans are the same size, thinner but usable, and a LOT cheaper... You could use a steel lasagne pan, just make sure it fits your kettle!
#3. Silicone basting brush and a steel / stainless steel cup for keeping sauces / mops warm / hot while cooking...
#4. Stainless steel spatula for flipping burgers.
#5. Tool hanger(s).
#6. Meat fork.
Additions I have found nice are...
#7. High temp silicone glove for grabbing / moving the grate if need be.
#8. Hinged grate. Not 100% necessary, but it certainly makes long cooks easier!
#9. Waterproof charcoal bin for coal storage.
#10. Condiment rack. Nice, works with some stuff, not so great with other...
Not going to mention charcoal chimney and lighter as you stated you already had them... but if not, but them under necessary.
I also like having a cutting board and prep knives out by the grill on a folding table. Paper towels, waste bin etc... But again, that is elaborating the art...