I decided to use some 30 AWG silicone insulated wire that would flexible enough to allow me to position the Neopixels easily in the tree. I wasn't sure how many would be enough, so I started with 6 and hopefully the spacing between Neopixels will work.
Here's the test string:
Not my best work, but it's Christmas Eve and I wanted to have lights for my wife for Christmas.
A quick demo of the lights on the tree - I need to tuck the wiring in a bit.
I'll finish the project in my next post. I've decided to add an ambient light sensor to adjust the brightness as suggested by beacon_dave . And I think that I'll also add a presence detector to turn the Neopixels on and off, but haven't decided which type. IR or Ultrasonic would be the easiest, but I might try using a time-of-flight sensor or possibly a Person-Sensor. The Person-Sensor might have problems with low light, certainly wouldn't work in the dark as it uses an image sensor (although it might work with IR illumination).