AirZound Bike Horn: I love having this on my bike (any type of bike). It's great for alerting motorists that are driving without looking. It fits in a water bottle holder (we just velcroed it to the frame) and you refill it with a bike pump.
Power Grips: These attach to most pedals to replace toeclips. I love that you can wear any type of shoe with these, you can adjust the size, you don't have to worry about getting stuck, and they usually cost less than $30.