What hardware to use?

Building an odometer, there's one overriding requirement: measure how fast the car is moving and/or how far it has moved.  For my use, I'm going to take a fairly universal approach: build a wheel sensor that signals rotations - most modern cars have a speed sender signal that can be tapped, but that's much more invasive than a simple wheel sensor. 
The standard among most modern rally computers is a Hall Effect sensor mounted stationary, and one or more magnets on the wheel hub - so that's what I'll be doing.  A little bit of research showed lots of success using the US5881 sensor, so I picked one up from Adafruit.

So, what about user input?  While I'm just prototyping this, simple push-buttons will do just fine - when I get to putting this into the car, I'll look into some panel-mounted buttons instead.  I picked up four (Up, Down, Menu/Enter, Reset/Cancel) buttons from FunGizmos.

So, that handles our input from the user.  What about output?

Well, we need to display the two odometers (trip and incremental).  Six to seven characters each.  Clock is 8 characters if we use seconds (or 1/100's of a minute), more if we need higher accuracy.  A 16x2 LCD would probably do - while I'm prototyping, I opted for a 20x4.  For simplicity in prototyping, I also am using an i2c/spi backpack.