General:
* Decorate the tree
* Adorn the tree
* Dress up the tree
* Enhance the tree
* Embellish the tree
More festive:
* Sparkle the tree
* Tinsel the tree
* Glam up the tree
* Add some sparkle to the tree
* Give the tree some bling
More specific:
* Hang ornaments on the tree
* String lights on the tree
* Put tinsel on the tree
* Sprinkle glitter on the tree
Ultimately, the best phrase depends on the specific context and the type of glittering you want to convey.