I don't do a lot of bike riding, but I plan on going to California soon and touring the state on a bike. I got to wondering the same thing as you about speed and distance, and I found speedometers and odometers for bicycles on Amazon. There is one for $20 that seems to be the best deal. I really liked it because it also showed the outdoor temperature. It also shows calories burned:
http://www.amazon.com/Schwinn-SW654-17-Function-Bike-Computer/dp/B000DZD3FQ/ref=pd_sbs_sg_2
There were a couple for $10 that had like 10-12 functions, but not temperature or calories:
http://www.amazon.com/Bell-1002294-F12-Bike-Computer/dp/B001FCFK5C/ref=pd_sbs_sg_2
http://www.amazon.com/PTI-Schwinn-12-Function-Computer/dp/B000068CJZ/ref=pd_sbs_sg_4
You attach the device to a fork on the front of your bike. Then, you put a magnet on a spoke that comes within 1mm of the device. You have to program the device with your wheel size like 20" or 26". There is also a cable to the handlebars where you see the readout. Good luck.