Grilling & smoking accessories...not so much toward tool
sets (most have stuff you don't really need, and some of the tools aren't well-constructed): a good, heavy built pair of long-handled tongs, long-handled spatula for grilling burgers (I prefer wide-faced tongs for all my grilling), then:
a digital probe meat thermometer (or 2, but 1 will do for starters...I currently have 3), with cable or remote (R/F) display is a must for larger meat cuts and poultry, IMHO);
charcoal chimney (if you use lump or briquettes, another must have);
smoke woods (hickory, cherry, apple, pecan are among my personal favorites) if you have a limited supply;
digital food or postal scale (very accurate) of 20-30lb capacity, if you plan on making sausages (fresh or cured), curing jerky, pastrami, canadian bacon, buckboard bacon from scratch (just to name a few);
electric coffee grinder, for grinding your own spices for dry rub recipes, brines, brine/cures;
insulated food handling gloves, for removing smoked meats, fish, veggies and taters from the cooker, pulling pork,etc (if you have good, solid tongs, you can use them for most things);
pair of wide meat handling forks (4 inch width or so) for lifting meats off grates (can replace gloves and tongs for heavy cuts of meat);
box of powder-free vinyl or latex gloves for food prep (raw meat handling, etc...looks more professional to guests, and shows them you really do care about food safety and their well-being...note: some folks are allergic to latex, so I prefer vinyl);
stainless steel grill toppers (no fuss clean-up, as it's hard to damage stainless with hand cleaning methods), or disposable aluminum (similar to expanded steel sheet) grill toppers...for specialty items like meat loafs, sausage loafs, chopped veggies, etc;
disposable aluminum or stainless steel steam table pans (for cooking sides, finishing ribs, brisket, pork butts, etc)...full-size (large) are approx 22" x 12", half-size (small) are approx 11" x 12";
a couple rolls of heavy duty aluminum foil...if you use it much at all, you don't want to make a mad dash to the store in the middle of a smoke when you're ribs, butt or brisket just hit the foiling point after just finding out your spouse or kids used it all up 2 days ago...;
That should be a good start...keep in mind what you would like to have for your own stash of gear and supplies for your outdoor cooking...stuff you keep an eye on and a mental inventory of, so you know what you have to work with that day, and what you need to get more of for the next round.
And, there should be much more from others, soon...took me a bit to write this up by now...
Hope you have a GREAT Christmas, and many successful smokes!
Eric