I'd have to agree that taking the us will eat into alot of your time in Canada.
As for Toronto, there are myriad things here that are great. I live about 40 minutes outside the city. I'm glad to hear you're coming 'round here as Ontario often gets overlooked and is definitely underrated.
There is something for everyone. Toronto is full of shows, museums, galleries, historic neighborhoods, markets, wonderful shops and restaurants, beautiful parks, everything. Sure, you can party all night and live it up, but you can also hit the sack at a decent time and relish what the city has to offer, especially with only a few days.
personally, I wouldn't miss Ottawa. It's a beautiful city and Jacques is right, it has an incredible small-town feel to it. There's so much going on there you'll be at a lack for boredom.
Algonquin will be cold for sure. Even when the weather warms up during the days, the mornings and evenings/nights are frosty in spring. Please don't let that deter a few days of canoing. In Canada, cold is a part of life. If you dress warm, the cold shouldn't be too bothersome. That's how we deal with almost 6 months of winter
Pack some thermal gear that's lightweight and warm. Do a little research and bring a good jacket, sleeping bag if you plan to camp. You could also just visit the area without camping and hit some rivers and lakes. there are SO many trip options you're not going to have to go deep into the bush if you don't want to. Let me know if you decide to go. I have some invaluable resources to tap into. This would surely be a highlight of your trip. It's one of the most beautiful places in Canada in my opinion and I've been coast to coast a few times.
PEI; you might be a little put off to find many thing closed on the island at this time of year. Peak season is the summer and many things operate only seasonally at that time. The upside is that PEI has about 125,00 population which swells to a million in summer. Though you may not find all the tourist haunts open in April, you will get one of the best PEI experiences by hanging with the locals. They are extremely friendly with fantastic senses of humour and a real welcoming spirit. Hit the island for a day or so at least. Do not miss Halifax and area. No worries about the time of year, you'll still have an awesome time.
Going to hit things in vaguely the order listed above, although not quite sure how I'm getting from the far east of Halifax to Vancouver yet blink.gif Going from Toronto straight to Halifax and working my way back might be more feasible?
This is always a bit of a hassle. Both Toronto to the east and Toronto to the west are long trips. With only three weeks, you're best bet is to fly. The west will be warmer and might be a good place to start. then fly to Toronto, check out the area, head up to Ottawa, a five hour trip, or so, you can do this by bus. You may find a cheap flight from T.O to Ottawa if you prefer. It depends. Sometimes this can be quite expensive. Canada's biggest downfall is how much it costs to fly around inside the country
Then fly from Ottawa to the east coast, tour around and fly back to Toronto to head out. A couple of weeks won't make a huge difference in weather for the eastern part but, you just never know here! There are cheap flights available, but its easier to find a cheapie from T.O. to the east rather than T.O to Van, that's for sure.
Okay, I've rambled on enough. If you have more questions, fire away! And have a wonderful time!!!