Karen,
I bought a really cheap one at Target. It is on my bike right now, which is with DH (long story). I know it is red and put out by Schwinn-hey it was inexpensive. It has 12 functions (these are the ones I recall)-current time, distance, odometer, highest speed, average speed, time spent out biking, calories burned, fat caloires burned, outside temperature. It has a backlight for when it is dark. It was really inexpensive (around $25), works great. It is not wireless, but it is not obstructive either. I have been very happy with it.
Edited-forgot to add: You preset it with your age, weight, height, wheel diameter and probably a few other things I am blanking on, which is what helps it calculate the milage, calories, etc.
Jennifer
“Live as if you were to die tomorrow. Learn as if you were to live forever.”
-Mahatma Gandhi
"We are what we repeatedly do. Excellence, therefore, is not an act but a habit."
-Aristotle