Generally, the one upgrade you can make to a bike that gives you the most bang for your buck is high-quality tires. It's also an easy upgrade to make in isolation from everything else. But if you're satisfied with these tires, no need to change them. Similarly, there might be a different saddle that would be more comfortable, although that takes experimentation. I'm not a fan of flat bars for anything beyond in-town riding, but changing to drop bars would get expensive because you'd also need to change your brake/shift levers. You could add bar-ends to give yourself a second hand position. The bars on this bike are pretty wide for riding on roads or groomed trails—on my wife's town bike, we've cut 5" off the bars to put her hands closer together.
I would not make other changes.