Proximety sensors
I would sugest looking at the SRF modules from Devantech. They have a very easy to use i2c interface. Ive used them for several projects so far. They use ultrasound echos to measure range. It has the ability to adjus physical parameters about the soundpulses that is send out. You can change the power, timing, etc. Works well for me at least.
Of course its always a good idea to have a welldesigned bumper in adition as a last resort. First design a sturdy bumper that can absorb a colision at full speed, and then add some microswitches or somthing to detect the impact.
Good luck,
Magnum