Grips are a must-have. The stock ones actually hurt, they're so rock-hard. Kuryakyn makes some that are probably a bit overpriced for what they are, but they're comfy enough. They make footpegs and floorboards as well (and some mid-controls if you think the scout controls are too far forward).

I have a national cycle deflector shield, not quick-detach but it suits the bike well enough and it works for how I need it to (at least with the mounting hardware I got, it's not hard to remove but requires tools and might be fiddly to re-attach and re-align).

I also got this set of handlebar risers: https://www.amazon.com/gp/product/B00M9IG488. They made the reach to the handlebars a bit more relaxed, but that depends on how long your arms are and what angle you want to be sitting at. The reduced reach handlebars are overpriced, and Polaris screwed us in regards to aftermarket handlebars (the grips are 7/8, and the bar swells up to 1 inch, so good luck finding aftermarket handlebars, and I don't know if I trust spacers to hold a 7/8ths bar in place- an experiment for another time).

I switched to a mustang seat, I prefer it but depending on how the stock seat does for you, it may not be so bad. It moves the riding position up and forward about an inch in both directions and I don't get saddlesore nearly as much or as quick as I did on the stock seat. If someone's charging you labor to swap out the seats, run, they are ripping you off. The rider's seat doesn't bolt in, it pulls right off and "pops" back in place, and the passenger pad has three bolts- if you can turn a socket wrench, you can swap out the rear seat. And if you don't have a passenger often, that one doesn't even need to be addressed unless you want to change the color of the leather.

You'll want some kind of luggage, at least to put your registration and insurance paperwork. There isn't even a cutout under the seat for it. Vikingbags has some saddlebags that I absolutely love (these:https://www.vikingbags.com/indian-scout-viking-indian-specific-brown-leather-motorcycle-saddlebags), but they can be a bit pricey (definitely cheaper than stock bags though, those are absurdly overpriced for what they are). I didn't get the quick-detach, though they offer that as a $150 option.

Some will say get a better exhaust, honestly I think even the stock exhaust is louder than I'd like, so really that's a subjective thing you'll have to determine on your own.

I haven't done an LED headlight, but this bulb works and is cheap, and a definite improvement over the factory bulb: https://www.amazon.com/gp/product/B000CR3UW6/