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.
Shimano WM51 bike shoes: I've been using Power Grips for a long time because I don't like having to change shoes. I finally caved and bought a pair of cycling shoes after doing research on foot problems during long-distance cycling. These have a solid sole (reduces stress on arches) and wiggle-room for my toes, while fitting comfortably on the sides. I was skeptical of the velcro closure, but they adjust to fit well, and make it easier to take these off quickly.