"Treats" is a very tricky term. It literally means "Food that humans give animals to make humans feel good about themselves". Just because an animal goes crazy over, and eats a bunch of anything...doesn't mean it is good for them!
I'm with JJ...Good GBS 24/7 is the best treat of all!
Pumpkin,gourds,winter,summer squash are all very nutritious!
Greens...anything in the brassicas species are very nutritious as well. A penny's worth of kale will substitute a nickels worth of oyster shell, but that's another story.
Fruits...most anything but avocados.
Lettuce...particularly Ice burgh lettuce, is pure evil!
Avoid white/wheat breads/overly processed grains because they are pure evil as well! They are not even fit for human consumption.
Meal worms....If you have them, then feed them...To adult birds as JJ points out. Just from an accounting standpoint, they are hands down the most expensive protein supplement on the face of the planet, other than kitten food!
Unless you are raising geckos, which need live food. Meal worms are good for them!