This is an adorable free outdoor extension to the nearby Toronto Railway Museum. There are a number of trains that are parked here that you can crawl around on, look at, pose in front of and generally hang out at. There's a train station where you can ride a small-gauge train, and a some grassy areas to hang out on, and places to sit in the shade.
This would be a great place to take kids, and it's a little oasis in the middle of the busy area between Rogers Centre, the CN Tower, the Aquarium and other downtown attractions. There's also a bike rental kiosk here.
It's a fun place for relaxing and people watching.