For fireworks you won't need a flash or fast lens. You will need a tripod.
For the parade and wedding receptions... there are not many flashes that fall in your budget range. Many of those won't do much more for you than the on camera flash does. If you could go a little higher to $300 the Canon 430EX is in reach and it's a nice entry level flash. My feelings are that if you can't afford a solid flash hold off until you can because it can be really frustrating trying to work with an underpowered flash that is not articulated.
I also tend to shy away from third party flashes (the cheap Sunpack, Targus, Bower and Vivitar types rather than the Sigma or high end third party flashes) because they can very easily cause power surges that will damage the electronics in your camera over time. It's the same reason you can't use old flashes with your DSLR. It's like using cheap batteries. I'd rather pay more for that piece of mind.
Another option for low light is a fast prime lens. There are a couple within your reach price wise and will help you get low light shots.
