Thanks to the wonders of the Arduino, there are a load of useful electronic modules available on ebay at ridiculously low prices. You don’t need an Arduino to use many of these devices, so here’s how you can add a buffer transistor to a PIR module to let it switch strings of battery operated LED lights.
The typical quiescent current of these PIR modules is just 50uA (50 micro Amps) so they will run a VERY long time on a set of rechargeable cells before they need recharged. The time they last depends on how often the lights are triggered.
This allows the possibility of an outdoor Xmas tree in the middle of nowhere that turns on whenever anyone walks near it and then turns off when they leave. Add a solar panel to top up the batteries and you could have a very low maintenance decorative effect.

As mentioned in the video, the modules may have a movable link that can switch between two modes. retriggerable and non-retriggerable. In non-retriggerable mode the lights will turn on for a fixed time and then turn off again even if there is continuous movement, but then re-trigger again for another time cycle. In retriggerable mode the timer keeps being reset every time movement is detected, meaning the lights will simply stay on while someone is in the vicinity. the retriggerable mode is the best in this application, and is set by default on many of the modules.