I use most of the below on both my kettles and WSMs.
Must haves (for me)
500’ HD Foil (Sams or rest depot), Chimney, welding gloves, food tongs, charcoal tongs, latex gloves for food prep, Instant read thermo.
Makes life easier
Half & Full steam pans (The SS ones and the disposable alum. ones) Large galvanized ash can (~30 gal), Metal ice scoop for reusing charcoal, silicon food gloves (I have the Raichlen ones) , sauté pan just for the grill, grill brush (I prefer the ones with replaceable heads), meat thermometers, Pam spray (2 pk of the larges 8 ozat Sams is only $5), a garden with fresh herbs, pizza stone.
Also for the WSMs I use the deep fry thermos in the vent to check temp. They are easy to calibrate and if the get dropped/broken/lost, I am only out a few bucks.