I generally have good memories of summer camp, though there were a few bad moments mixed in. We were fortunate that there was a large nature preserve not far outside out of town, and that's basically where I went to organized camp. One year I did a week of day camp focused on pioneer life and one year it was swamp explorers. On top of all this, I was in Cub Scouts and then Boy Scouts, so I did a lot of camping on weekends and generally went away with the troop for a week at a time. There are some away camps here, but they seem a little more involved, and we aren't quite ready to send our son away on one of them (but maybe next year).
Anyway, Toronto does have a lot of themed day camps, though I would say that they lean towards arts and theatre/music or technology and much less on just being out and about with nature. That said, Harbourfront does have canoeing camp and even a camp focused on learning to sail. I don't think my kids have quite as much interest in nature as I did as a kid, though I'll check in again next summer and see what appeals to them.
I came dangerously close to not getting them into any camp at all, but in the end I signed my son up for two weeks at Harbourfront (he took the last slot in fact) and my daughter for one week at the Design Exchange. What I didn't quite think through is that both of these are relatively close to my workplace, which means I will be responsible to get them to and from camp, which means a solid 3 weeks I can't do any biking at all! Not ideal. I guess it is even more important that I go to the gym to make up for all the missing exercise. Actually I should look into buying a transit pass for July, which I certainly hadn't planned on doing. If they enjoy themselves (and avoid being quite so bored at home -- or at least so vocal about being bored!), then it will have been worth it.