OK you're gonna have to drop the vaguarity and get straight to the details:
What is the application. That will say a lot of things- how fast, can you touch the target, is optical going to be applicable, etc.
How far off do you need to measure
How accurate do you need it to be
How advanced can this become- I mean a stereo camera and DSP system might do some neat things but probably a waste of time to discuss if you can't do anything like this.
It's probably worth mentioning that measuring distances is rarely a simple task. Ultrasonic works, a fairly advanced project though and is only applicable for large, flat targets at short to medium range, with a fairly limited accuracy. The optical parallax methods work but there's a lot of issues including getting the right hardware to do it, and the electronics may be relatively advanced.