Hybrids are versatile. I'd ride it a while before swapping out the stem and handlebar to see what kind of riding you want to do with it.

You may want to try a trekking or a butterfly bar. They're inexpensive and will work with your existing brake levers, shifters, and stem. The biggest downside to flat bars, IMO, is the lack of hand positions. The trekking bar fixes that problem by giving you multiple hand positions.

I'm using them on my 1993 Trek 750 and I've been happy with them. I do need to get around to installing a rack on the bike one of these days.