Grilled chicken (diced) mixed with shredded lettuce, cucumber, tomato, pinch of onion, teaspoon or two of sour cream with dill, salt and pepper mixed to taste. Combine the above well and stuff into your pita.

Put in mustard, two fried, all beef hot dogs, round fried potato slices, sauteed onions, sauteed peppers and call it an Italian Hot Dog on a diet.
The Bear

I love pitas! I like to fill them with chicken salad that I make with celery, green onion, dried cranberry, mayo, salt, pepper and curry powder. Usually add some lettuce.

I also like egg and salmon salads. Hummus with mixed vegetables is great too.

Grilled or roasted veggies is my all time favorite.
Close second is salad.

I like greek salad in a pita, as well as all the usual schwarma, kofta, falafel. Left over chicken or lamb curry is good in pita.
I bet RubyRose has some good recommendations.