Here are two helpful links to make sure your LEDs live long productive lives. They're actually about using LEDs as computer caselights, but there's plenty of meat about wiring and using an inline resistor to make sure you don't kill the LED with thermal runaway.
A normal LED caselight
A Luxeon Star caselight
I'm uncertain how visible a normal high intensity LED will be across the field. The Luxeons are MUCH brighter and bigger, so you're more likely to see them. They can draw up to 5 Watts at 7V, though, so I'm thinking running off the digital IO might be a bad idea, as I'm not sure what those are rated at. I'm doubting many sensors draw even 1 watt of power when operating, though.