Your proposal sounds very good. In what month were you planning to bike there? Here are a few comments on the routes/destinations you mentioned.
1. The route to St. Peters could be done by road or on the Confederation Trail, an abandoned, re-surfaced rail bed. On the highway, I recall a generous shoulder most of the way, so the cycling would be safe, even though it's a main route. It's reasonably flat, too. You could easily loop back to Charlottetown via smaller routes, though there will be some hills east of Mount Stewart. That would be a long day in the saddle.
2. I'd suggest biking out of Charlottetown on Hwy 2 just past Dunstaffnage, then take Hwy 6 to Grand Tracadie and into the National Park. The ride past Dalvay to Rustico Island would present wonderful views and great stretches of beach. Then return via Brackley on Hwy 15. This route would be reasonably flat.
3. The route to Argylle Shore via Canoe Cove (19, 19A) is also very nice. There are a few hills, but views of the Strait would be great on a sunny day. I haven't biked Hwy 19 from Canoe Cove to Rocky Point and back to Ch'town, but it ought to be equally splendid.
4. The central part of the Island (Hwy 2 to Hunter River, for instance) can be quite hilly, although you could take the Confederation trail if you had a mountain bike.

Biking on PEI you are always aware of the wind. It can be punishing, so you might want to choose your routes according to wind direction and minimize the amount of time you're biking into it.
There is a bike repair shop in Charlottetown: http://www.macqueens.com/shop.html Once they rescued me near Borden when my wheel pretzeled, drove me to Ch'town, and built me a new wheel. I haven't been in their shop in many years, but I won't forget their service.
A bike trip to PEI is very worthwhile. I've biked there on many, many trips and have had some great days. Hope it works out for you. (You could also use Summerside as a base. It's easier to get in and out of by bike and there are some very good, scenic routes close at hand. Let me know if you want info on these routes.)

Thanks geebee. I'm greatly appreciative of all your suggestions; you make this trip sound totally worthwhile! I'll probably be doing it June or July, something likely in there. As for Summerside, I was thinking Ch'town might be a bit more interesting for the non-bicycling days but I'll read up on Summerside and let you know if that idea appeals.

Sometimes it blows my mind what a terrific resource this site can be... how some traveler out there KNOWS and has experience with one's travel dreams. Geebee, thanks again for sharing your knowledge base!

All the best, Daniel

You're welcome. Yes, Ch'town offers more on non-cycling days and if it's your first trip to the Island, you should stay there. In June, the tourist season hasn't really started, so the roads are less busy and some places may not be open. However, the weather can be iffy, too. Good luck and enjoy.

The best source for Atlantic Canada cycling:
http://www.atlanticcanadacycling.com/cyclingdata/pei/index.html

I only cycled across the island from the Big Bridge to the park, through the park, down to Charlottetown to finish at the Nova Scotia ferry. Here's a rough map: http://tinyurl.com/lvm4wf

It was all good except for highway 6 between the western and eastern park of the park, which is narrow, hilly and fairly high trafic.

I missed the bike trail on my way back from the park so I cycled highway 2 with high traffic but wide shoulders. There were a nice market The trail surface is rather coarse but not too soft. I had 50mm tires so it was a piece of cake. I think 35mm should be enough for a long trip on the trail. 28-32mm are ok for a shorter ride.

Cyclists don't pay the park's entry fee.