This 265-acre zoo is the country’s biggest and oldest, with over 6000 animals and re-created habitats from around the world, from African plains to Asian rainforests. It’s deservedly popular, with especially large crowds on discounted Wednesdays and weekends in good weather, and any day in July or August (try to go Monday morning). The southwest Asia Gate (four blocks north of the West Farms Sq–E Tremont Ave stop, up Boston Rd) is your easiest access point by subway.
Once here, your best plan is to grab a free map and find out when and where the various animal-feeding sessions and demonstrations will be held that day. The full experience ticket (10% cheaper if purchased online with a code available on the website) allows access to all of the zoo’s attractions. This includes the wildly popular Congo Gorilla Forest, a 6.5-acre exhibit with gorillas and exhibits explaining conservation projects in the Congo, and the Wild Asia Monorail ride (May to October). Note that the pay-as-you-wish deal on Wednesday is only for general admission and does not include most of the popular attractions, but it's only $10 for a full-experience ticket that day. Note that some attractions close November to March.
Opened in 1899, the zoo has done many good works. In the early 20th century, for example, its collection of bison were used to restock the wild population in their native Great Plains area.