HAL has a buffet as well as the dining rooms; for a 7-night cruise there will be 2 formal nights as you described, and the others will be informal. Khakis are quite appropriate for the informal nights.
As far as whether the entertainment is "good" depends on your own personal likes and dislikes. Like the other big-ship cruise lines, they have one or two "production" shows in the big theater per cruise, plus lots of individual entertainers in the various lounges. Games and contests for guest participation are also offered.