A flash will give you a lot of additional opportunities to improve snapshot pictures to better controlled more proffessional look. But you need to get a decent one with good features. I don't necessairly mean an expensive one. But one that can be used off camers, tilt, maybe swivel. Also one that you can manualy adjust. There are off brand flashes with these qualities and will not break the bank like the Nikon and Canon top models do. A cheap one will not be much of an improvement over the onboard flash. And just be wasting money. Nikon SB-600 is about $180, not sure about other off brand ones.
A good sturdy tripod is a must for pictures that need slow shutter, or dark scenes.