I've been known to spend five to six hours inside the Royal Ontario Museum because there is plenty of exhibits to see. I thoroughly enjoy the museum's World Cultures Gallery and the Natural History Gallery.
The World Cultures Gallery houses art and architectural history from Asia, Africa, Canada, and the Middle East. My favorite exhibit explores the ancient architecture of China using artifacts and miniature models. Included in this exhibit is a full scale replica of the interior of an ancient palace from the Forbidden City. You can find lots of sculptures and deities based on the Buddhists of early China as well as the history of early Asians in Canada.
If you like bats you'll really enjoy the Bat Cave located within the Natural History Gallery. In addition to the flying rodents several species of snakes and giant spiders are also on display. Most of the displays dealing with animals are featured as fossils or skeleton replicas thereof. The dinosaur exhibit, as most are, is quite interesting.
The museum also features touring and temporary exhibits. My experience here is that the museum is rarely crowded which allows me to explore the exhibits freely without having to worry about getting in the way of other museum patrons.