I entered Canada at around 10 PM at Sault Ste Marie, ON, on July 5th. I was in Canada all day on the 6th and 7th. I was there half of the day on the 8th at Niagara Falls. Here was the weather pattern for me.
July 5th (night)- rain
6th- rain all day
7th- no cloud in the sky all day
8th (morning)- rain
As far as the temperature goes it was 70's the whole time (perfect). There were no mosquitos, biting knats, etc. The farthest east I drove was Ottawa. I did do some Quebec driving from Pembroke, ON to Gatineau. The day I was in Ottawa was the 7th and it was the most perfect day. I could actually see myself living in Canada or northern Michigan in the summers one day.
I would think that June would be pretty similar. As far as rain goes it is pretty similar to the midwest and eastern US. It's more of a hit and miss thing. I always seem to run into heavy rain at some point on my road trips. June-August is definitely the best time of the year to visit Canada.
I'm sure you already know this, but the bridge to PEI costs $46 CAD ($33 USD, cost is for round trip, you pay when you leave the island). I remember that being a jaw dropper when I first read that.
If you wanted to get really adventurous you could take the ferry from Fortune, Newfoundland to Saint Pierre and Miquelon which are three islands that are actually part of France. I think the ferry ride is one hour long. It costs $93 CAD (67 USD) to take the ferry (round trip), but if you want to go to France while you're up that way there's your opportunity. FYI it is a passenger only ferry and parking for your motorcycle costs $10.