I think "Let's go to the movies today." should also be accepted. "Let's go to the theater today." sounds very British, as an American I would never say that.
These exercises are very inconsistent with the use of the word "together." In many cases, it is simply implied, but in other cases, it is actually necessary as part of the response. :(
For some reason, there was no option for "Let's go to the theater today." Instead, all I could type was "Let go to the movie today." and got marked wrong