What's an inexpensive ultrasonic range sensor that's reasonable waterproof or weatherproof that also interfaces easily with an Arduino?
I've used these kinds of ultrasonic range sensors, which are cheap and trivial to wire to an Arduino, but they're not very robust and are easily damaged since you can't cover them without that covering obstructing the soundwave.
Maxbotix sells a "Weather-Resistant" ultrasonic range sensor, but it's around $100, which is a little out of my price range.
I've researched those "parking sensors" used on cars, but I haven't seen anyone successfully interface them with an Arduino. It's also hard to tell how they're controlled. Some sites sell batches of the sensor for $3/each, but I don't think this includes the control electronics. Ebay listings that include what appears to be a bulky controller board seem to go for around $50-$100, which makes them a little pricey.