Toronto for a day or so?

Going where and from where?

I am in Ottawa, and my son lives in Toronto, but I am lame at recommendations. It's been a long time since I went there with a teen. CN tower is always recommended, unless you have a thing about heights. The observation deck floor is see through. Toronto Island is supposedly the place to go for picnics, beach and swims. We've had our first heat warnings, so though cooler next week, it could be quite hot depending on when in June. And if he is into baseball, maybe there is a Blue Jays home game.
https://www.timeout.com/toronto/things-to-do/best-things-to-do-in-toronto
https://www.atlasobscura.com/things-to-do/toronto-ontario
https://www.destinationtoronto.com/things-to-do/

Coming from Niagara Falls, on way to Wisconsin.

There are some nice wineries on the drive between Niagara Falls and Toronto, it's been a few years since I made the trip so I don't recall the names, but we had a delightful lunch at one. This was in August and it was just beautiful. Once in Toronto we stayed at a downtown chain hotel and did the Tower experience, also wandered around the baseball park. It was a college trip so slightly older kid.

I just got home from four nights at the Sheraton Centre on Queen. Love that place, but its not inexpensive.

Go to Niagara-on-the Lake for the day, the best little town in North America. Walk around, shop, catch a play:
https://www.shawfest.com/

If you are extremely lucky you might see me.

Hockey Hall of Fame. So fun. And CN tower. Tour of Blue Jay's stadium

Not sure of the 15 year old interests but RIpleys Aquarium (if its open late go in the evening), CN Tour, Hockey Hall of Fame are good choices and could all be done in one longish day. My kids also really like the ROM (Royal Ontario Museum). And a Jays game is fun especially if you are baseball fan. There are a number of smaller hotels east of Yonge, south of Bloor down to Lakeshore that are often more affordable than the large chains. I really like NOTL but it is very busy in the summer and not sure how much fun your 15 year old would have visiting wineries. Its a nice drive along the Niagara Parkway from Niagara Falls to NOTL. The Whirlpool Jet Boat Tours are a lot of fun as is the Maid of the Mist. Have fun!

The Royal Ontario Museum is great if you are interested in places like that (I am). I also found the Fort York National Historic Site really interesting. That might be a better choice for part of a single day in Toronto with a teenager.

I love Toronto but it's been years since I've been. I used to love to walk down Queen Street, with its funky shops and eat in Greektown or Chinatown. I do think the above recs for Ripleys, CN Tower, The Royal Ontario Museum are spot on. I would also suggest Castle Loma, and maybe a trip out to Toronto Islands. There's also St. Lawrence Market, which is a great place for food!

To make the most of your very short visit, study the TTC -- Toronto Transit Commission -- with its network of subways, streetcars and buses. The subway itself may be a tourist attraction for a teenager, and even you.

https://www.ttc.ca/en/

We went there for a baseball game weekend. We stayed at the M arriott hotel which is built right into the Blue Jays' stadium and it was great to be downtown, in the middle of things. The CN tower is right next to it, and the waterfront nearby. This was one place where we thought the Hop On Hop Off bus was well worth it and the tickets included a cruise on the lake. Broadway market was a highlight. Great city.