Since this is an outdoor project, you will need bright LEDs, otherwise they just won't show up in daylight. Look for "high efficiency" and/or "high brightness" LEDs and test them in daylight before committing to a particular type.
You may even need to go as far as using the big 1 Watt (or more) LEDs designed for lighting applications (made by Cree, Luxeon, Philips, Osram) rather than the smaller types designed for indicator lights.