I'm lucky enough to live in the great city of Toronto. The weather will hopefully be around 60ish, but like most cities on the east, we've had a pretty nasty winter!
As for recommendations:
CN tower ( super touristy but really cool)
Ripley's aquarium (right at the bottom of the CN tower, just opened this year)
If you're into sports, you could watch a blue jays game. Hockey will be in playoffs then, if the Leafs make it, it will be very hard to get tickets.
If you're into arts and culture...check our the Royal Ontario Museum or Art Gallery of Ontario
There's also a lot of great theatre in Toronto.sadly, it looks like you'll be here just before the lion king comes to town!
Toronto is also one of the most multiple cultural cities in the world, so check out: Chinatown, greektown, koreatown, little Italy, little India, little Portugal and on and on...so much food and great people!
Toronto.ca
Niagara Falls:
The falls is great, we usually take the hour drive at least a couple times a year!
Journey behind the falls- really cool you actually go right behind the falls and see the rushing water right in front of you! You can also stand right beside the falls and feel the mist ( they give you a poncho) this may be a bit cold in April...
Clifton hill- it's kitschy but if you like fun things like wax museums, haunted houses, hall of mirrors, then this place is for you!
Casinos- if you think gambling is fun. They have two very close together: fallsview is up by a lot of the hotels is the newer one. Casino niagara is right off Clifton hill. They're pretty much the same in my opinion.
Wine- niagara wine is world famous. So if you're into that stuff, there are lots of wine tours you can take (most hotels even have wine tour packages)
As for hotels, there are ALWAYS great deals on deal sites for the falls which include extras like a wine tour, restaurant credit, casino credit etc. for cheaper than rack rate!
Check out: travelzoo.ca, groupon.com, onespout.com
You can also find Toronto hotel deals on those sites, but they don't usually have all the extras like the niagara ones.
Hope that helps!