At the terminal, there are 3 different ferries, going to different parts of the island (although off-season they will sometimes combine service to Hanlan's Point and Ward's Island). I recommend taking the Hanlan's Point ferry, then walking the length of the island and taking the Ward's Island ferry home. This provides two main advantages: 1) as Hanlan's Point is the furthest west, it gives you a nice long ride along the harbour providing you with the best views of the city, plus as the dock at Hanlan's Point is very close to the island airport, you get an up-close view of the planes taking-off and landing, and 2) walking the length of the island is a great way to see it. The distance, depending on how much you meander and explore, is just over 5 km.
Each section of the islands has a different personality. I love the tranquility of Hanlan's Point. There are big green spaces for picnics, playing ball or just relaxing, and as most families flock to Centre Island, it's usually pretty quiet. The main path offers lovely views of water and wild, as well as some great glimpses of the city. Many of the trees, including the lilac bushes (my favourite!), are huge and if you're interested in trees, you can pick up a brochure for a self-guided tour to trees across the island, or print out a copy from The Canadian Tree Tours website.
The most lively part of Hanlan's Point are the beaches. There is both a clothing optional and clothing mandatory beach and they are big gorgeous stretches of sand, with the bonus of a view of the city. If you have concerns about the water quality, you should know that Toronto takes part in the international Blue Flag program, which is administered by the Foundation for Environmental Education in Denmark and which sets international standards for clean, safe beaches. The City had to be evaluated against 27 criteria and Hanlan's Point beach, as well as Gibraltar Point, Centre Island and Ward's beaches were all granted Blue Flag status (plus some other city beaches not on the islands). Water samples are taken daily at each beach to ensure continued safety, and results are posted online - so check the site, then go ahead and jump in!
As you get closer to the middle of the island, it starts to get more built up. You'll pass the Gibraltar Point lighthouse, the oldest surviving lighthouse on the Great Lakes, as well as the Gibraltar Point Centre for the Arts, the water filtration plant, and the island school. On your right, sticking out on the lake side, is the Pier, which I just love. If you have the time and the energy, you can detour inland and cross over to Centre Island. Centreville, the amusement park, is the big draw here, especially if you have kids. Even if you don't, I highly recommend taking the Sky Ride. It gives you a bird's-eye view of a small sliver of the park and it's relaxing and fun. Right next door to Centreville is Far Enough Farm, one of my favourite places on the island. There is a limited number of animals here but it's a great space. Like Riverdale Farm and High Park Zoo, it was facing closure due to budget cuts, but acceptance of a recent donation by the operator of Centreville will keep it open awhile longer. There has also been an offer to match funds from private donations, so if you'd like to make a tax-deductible donation, visit the Parks and Recreation website. If you happened to bring a picnic along, Centre Island has some of the best spots for picnicking with a view of the city.
If you want to continue your trek to Ward's Island, you'll have to retrace your steps back to the lake side of the island, as there is no connection between Centre Island and Ward's Island (or, if you're tired, you can always head for the Centre Island ferry dock and return to the city). If you continue your walk, you will soon have a choice to take the main paved path to Ward's Island, or veer off to the boardwalk. While both have their appeal, I generally prefer the boardwalk, especially on hot days - there's lots of shade and usually a wonderful breeze off the water. You'll know you're getting close to the end of your walk when you begin to see houses - Ward's Island has a small but vibrant residential community.
If you have some time to kill before the ferry, consider stopping in to the Island Cafe, just steps from the dock. They now have a liquor license so you can celebrate your visit to the island with a nice cold drink.