Ditto to a cover, I have one of theirs and it's nice and fits perfectly. Be sure to check the size though because they make at least two.
Not Weber related but just as, if not more important than the grill, a Thermapen and a some sort of monitoring system with probes ( I have a Maverick ET-733 which is on the cheaper side but has worked flawlessly for me so far )
The above are the two single best purchases I've ever made in grilling, not counting the grill.
Of all the accessories I have for my Weber, charcoal rails get the most use. For setting up an indirect fire they are indispensable, and only about $10.
Mine is also out 50* from food level.That lid thermometer may or may not be accurate. The one in my Smokey Mountain differs by about 30-50 degrees From what my Maverick reads at the grate.
Their Chimney starter is nice but pretty pricey; I love my Weber grill. When you get there you might see some other starters for comparison. But I might have seen the brand I can't remember at Home Depot.
The other chimney the HD sells is cheaper quality- the Weber is only a little bit more. The other one has a hinged release on the coals- that is something that is going to break over time. The Weber is easy enough to turn over and dump.
Jessy, get a nice pair of leather gloves. The chimney's are hot, hot.
Congrats on the Weber, they are a fine bit of kit!
Besides the chimney, I would also get a lid holder, that way you don't have to put the lid down on the ground and risk denting/chipping.
This would be redundant for me and probably most other Weber owners. My grill and most kettle-style Webers have a hook on the underside of the lid (vent side) that allows me to hang the lid securely on the side of the grill, no accessories needed.