KAC 300m rear sight used on Fleabay, a stock with a better cheekweld than the standard, and maybe a few buffers/springs to see if you can make everything as smooth as possible. This is assuming you have a decent trigger pull out of the gate. I have a PSA that has almost no take-up, a crisp break, and no overtravel, so those were my first upgrades. I decided I didn't like the A2 sight so I got a free float rail with flip-ups next.
If the trigger sucks, get the cheapest rear sight you can find(used on the EE, eBay, wherever), and a Geissele trigger. I would suggest the SD3G, but most guys on here will tell you to go with a two-stage trigger.
A sling would also be nice, but by then you're way above the (already blown) budget. Every time I set a budget for myself, I end up spending more just to get exactly what I want, so unless you want to waste money on something you'll switch out in the future, you'll have to choose wisely with $250.
After that, you'll need an optic for when you decide to go long range. Then a bipod. Then a free float rail. Muzzle device. Grip. Weapon light since there's the chance of using it in a home defense situation.
But then you think you might want an AR pistol or SBR for home defense instead. You build that, taking some of the parts from your current 16" Spike's. Maybe you'll decide to go 300BLK. The list goes on, AR upgrades really are a gateway drug.