A few things we did add via the dealer and that we like:
* Fantastic Fan vent covers so we can leave the fans open all the time without fear of rain
* Flamma bike rack on the back of the trailer. We just love it - super easy to use, as long as you're riding conventional bikes.
* Voyager rear view camera.
* A Tire Pressure / temperature Monitoring System (TPMS) (we had the dealer install a repeater to ensure the signal reached our cab)
Agreed, don't go nuts buying stuff before you go camping a couple of times. Bring along the things you'll obviously need, and feel free to start with disposable dishes / utensils until you figure out what you really want in the trailer. Keep notes of the "Ah, I wish we had a ____" moments and see what comes of that.
When you do buy something for your trailer, make it count - it should be something you'll use pretty much every time you go out. We started out with a Keurig that we inherited, but then we found our classic stainless LL Bean percolator that we'd had pretty much forever. With good beans, the right grind and just a few minutes of attention on the propane stove, it makes absolutely fantastic coffee. We make delicious coffee anywhere, anytime, with zero electricity and less wasted space. It turns out we don't need an outsized inverter or shore power to get our camping days off to a wonderful start. The perc pot is also great at heating up tea water between rounds of coffee.
An estate sale can be an amazing cost-effective way to quickly get a bunch of stuff you're going to use all the time: dishes, silverware, pots/pans, knives, and the like. We pretty much emptied my parents' kitchen into the RV when we closed up their house for sale, and that has worked out quite well for us. You could achieve the same at any estate sale. OTOH, if you prefer to add "permanent" items one at a time, there are incredibly cute things you can add, such as melamine "happy camper" plates and cups, GoVino or stainless steel wine glasses, insulated tumblers, stacking camp pots/pans, etc. The list of awesome, cute and retro opportunities goes on forever. Just go slowly and be sure you're really going to use them on every trip. Also, keep track of what you use and what you don't. Emergency equipment aside, if you don't use something on two camping trips in a row, consider ditching it or leaving it at home.
We see lots of Airstream photos with beautifully decorated interiors...and then we realize that such a space would likely require an hour or two of set-up and tear-down at every stop. So we've stayed with a relatively Spartan look. No table lamps or tchotchkes that require getting out and putting away at each camping stop. The trailer already looks great, so we don't feel like that's needed given how we camp today. That said, we do like to add colorful touches via bathroom and kitchen towels, a throw rug on the floor, and retro placemats on the table. Once we wear out or get tired of the AS-provided comforter, which is nice but very plain, we'll add a touch of color there, too.