Most TS you are allowed to get, soda, hot or iced tea, coffee, as your main beverage.
We have gotten milkshakes at Sci- Fi as our main beverage and/ or dessert.
When our DD's were younger ( both now over 21) a few TS would let them have virgin strawberry daquiri's- Cali grill, Narcoosee, Ohana's, Chefs of France, and at Tutto. But we were decline at Biertergarten, both resturants in Japan.
If you order tea with your meal (lunch and dinner), and then want coffee with dessert, you most likely will be charge for the coffee. For us it has been about 50 - 50. When we were at Flying Fish in Dec. we were not charge for coffee with dessert, but at Cali grill on the same trip we were charged.
At breakfast sometimes you can get a smoothie as your drink. Which for us was very good, both DD's would be ready to hit the parks and would hardly eat anything, but we could take the smoothies with us and they would have then drank by the time we got to the park. Also when ordering drinks at breakfast be mindful. That in some of the TS you can order, juice and another drink coffee, hot tea, or soda. At Donalds, we both ordered frunch and then a soda as well we were charge for it. But when we were at CP we ordered the same but were not charge for it.
I really think it depends on who you get as a server. But if you get a little bit of extra pixie dust make sure you let your server know you appreciate them, by adding a bit extra in the tip.