What are the most worthwhile things to do in Toronto on a first visit?

The CN Tower is a working antenna that happens to sell tickets. The Canadian National Railway built it on the former Railway Lands to fix broadcast reception in a forest of new skyscrapers, and it opened to the public on June 26, 1976, at 1,815 feet (553.3 m) — the world's tallest free-standing structure, a record it held for 32 years and the Western Hemisphere title it still holds. The levels stack by nerve: the LookOut at 1,135 feet (346 m), the Glass Floor at 1,122 feet (342 m), which opened in 1994 as the first glass floor of its kind in the world, the SkyPod at 1,466 feet (447 m), and since August 1, 2011, the EdgeWalk, a hands-free walk around the main pod's roof at 1,168 feet (356 m) on a 5-foot ledge, the highest full-circle walk of its kind. More than two million people a year go up.

The antidote to the tower is 13 minutes offshore. The Toronto Islands are a chain of 15 islands running about 3 miles (5 km) across the harbor, a 332-hectare park reached by city ferry from the Jack Layton Ferry Terminal at the foot of Bay Street. They form the largest urban car-free environment in North America — the only engines belong to service vehicles — and the three landings divide the day: Centre Island for the amusement park and beaches, Hanlan's Point for the western shore, and Ward's Island for the cottage lanes of a year-round community of about 600 residents. The skyline view back across the water is the city's signature photograph.

Stay east for the third act. St. Lawrence Market has traded on its site since 1803; the South Market hall holds more than 120 vendors, the North Market hosts a Saturday farmers' market, and National Geographic named it the world's best food market in 2012. A short walk east, the Distillery District preserves the Gooderham & Worts complex, founded in 1832 and by the 1860s the largest distillery in the world: 13 acres and more than 40 Victorian buildings, the largest collection of Victorian-era industrial architecture in North America, designated a National Historic Site in 1988. Distilling ended in 1990 and the site reopened on May 22, 2003, as a pedestrian-only village of galleries, theatres and restaurants.

From the boardwalk on Ward's Island the downtown towers stand in a single row across half a mile of harbor water, close enough to read the CN Tower's pod, with nothing in front of them but gulls, sailboats and the ferry cutting its white line across.

The CN Tower did not lose its world record to a finished building. On September 12, 2007, the still-rising Burj Khalifa passed 553 meters partway through its own construction — Toronto's 32-year title ended at the hands of a crane-topped concrete frame in Dubai.

• Morning — Start at St. Lawrence Market as the vendors open, then walk east into the Distillery District before lunch.
• Midday — Go up the CN Tower on a reserved ticket while the haze is thinnest and the lake breeze is up.
• Late afternoon — Take the ferry to Ward's Island and walk the chain west as the light warms, riding back from Centre Island after the commuter rush.

Summer weekend ferry queues at the Jack Layton terminal can stretch around the block; tickets are sold online and buying ahead is the difference between a 13-minute crossing and an hour on the pavement. EdgeWalk is seasonal, weather-dependent and booked in small groups, so it is the one item in this answer that cannot be left to the day itself.

Eat inside the route: the food question in this group is built around the Market where this walk begins.