Toronto native here -
Niagara falls: super touristy but a lot of fun. You can walk all the way along the falls which is pretty cool. Clifton Hill is where all the "tourist" attractions are - boston pizza has a huge arcade. Food wise you have mostly chains however there is an Italian place on Lundys Lane called Mick and Angelos, it was on diners, drive ins and dives and has really good food & massive portions. I would do maid of the mist - most people consider it a must do. I would skip the skylon tower, especially if you're heading to Toronto and can do the CN tower. If you head to the casino's I prefer fallsview over casino niagara - its newer and nicer. There's a big outlet mall called the Outlet Collection at Niagara Falls just up the QEW - worth checking out if you're going to shop, especially considering the US dollar vs Canadian.
As for Toronto
Since you're going in June I would try to hit a Blue Jays game if you can - great atmosphere and tickets aren't super expensive. I would do the CN tower if you've never been before. There's a Ripleys Aquarium at the base which is pretty cool too. If you want to shop hop on the subway and head up to the Eaton Center - it's massive and right downtown. St Lawrence Market is one of the best food markets in the world and has some unique vendors. If you're looking for museums, the ROM is pretty cool and not massive (compared to other museums). The Distillery District is really unique and has some awesome shops and good restaurants. Nathan Phillips Square is cool, it has our Toronto sign from the Panam Games. You could also take the ferry over the Centre island - it would take up a decent amount of time though. If you're into beer we have some awesome distilleries in the city! In terms of food you can get literally any kind of food you want - if you're looking for any recommendations feel free to shoot me a message, I have lots of favourites

. Take the subway as much as possible, the yonge-university-spadina line will take you into the downtown core and most places you need to go - cabs & uber are also readily available. Theres lots of underground parking or "Green P" Lots - they run about $20 for the day. There's no shortage of things to do in the city.
Anyways feel free to send me a PM if you need any help planning!