1) welding blanket: year round, one of these drastically reduces the amount of pellets you'll need.
2) Thermoworks thermometers. Best thermometers around. Thermapen One for instant read at time of taking meat off (most useful thermometer you'll have in your kitchen) and stay-in thermometers during the smoke (one for the meat and one for temp tracking inside the Traeger).
3) smoke tube. use to get additional smoke. Not a life changer, but nice.
4) bread pan for water. if you're in arid west, this helps keep meat moist.
5) plastic bus boy. These are the coolest things ever invented. They're food safe. I use to transport meat to family events. Surprisingly useful in all aspects of life: harvesting vegetables, starting garden seeds, harvesting honey, etc. Every family should have at least one set of bus boys. Readily available at Costco and Sams.
6) Aluminum foil from Costco/Sams. The super large one. I use for meat, making my own liners for drip tray (you don't need the commercial drip tray), etc.