We just have chocolate syrup milkshake mix in the house to add to milk, for both kids. DD#1 doesn't get thirsty (she's 9.5yo) so the only way to get her to drink is by schedule. DD#2 drinks when she is thirsty. We used to get chocolate powder or milo but DD#1 has issues mixing it in, so syrup worked better. We get it in 3L commercial sized bottles, if that says anything ...

There is no need for milk to get enough calcium
The obvious example being cows. Who don't drink milk. Followed closely by chickens, who make a calcium-enclosed egg every day.