A good tune-up by a conscientious mechanic at the local bike shop could be a smart buy
The typical sports store leaves something to be desired in the assembly and tuning. So you stand to gain better performance, possibly additional safety, and fewer problems in the future.
Aside from that, some bar-end extensions (if you don't have them) and a waterbottle & holder, or preferably a water-pack like a Camelbak, would be practical upgrade items. And of course, a helmet.
If you're going to be riding in the dusk or at night, consider a headlight and a blinking taillight (I like the Cateye HL-500 and RR-LD500). If you don't have a small multi-tool, a spare inner tube and a small pump, add those to the list of possibilities
The type of bike you describe is not really intended for high-performance off-roading, and some actually come with disclaimers saying they're not for off-road use at all. My suggestion is to focus on accessories that you can migrate to your next bike, or which are for the motor (Camelbak, gloves, helmet). Hope that helps