The Rothesay and the Conestoga are both very good sites for brand new divers, nice little wrecks to explore, not too deep (30') so they should afford you some good bottom time.
Last sunday we went for a little quad dive day in Ontario (it's a long drive, gotta make it count). Did Lock 21 2x, went up to Brockville had our tanks filled and did the Rothesay and Conestoga on the way back. We had the tanks filled at Dive Adventure Centre (big bright painted house on water st), it took about 20 minutes, friendly staff.
About Lock 21, depending on how comfortable you are in the water Lock 21 may be a bit much, there's strong current and vis isn't that great. You could always just stay on the railing and lines (there's bunch of em on the site) but I find that half the fun of Lock 21 is doing the drift (the other half is going back up to do it again). Be aware the line at the bottom of the lock that is suppose to take you back to shore is hard to find (that or I just suck at finding things underwater).
The Connie and the Rothesay are very easy and you should have a great time, if you can try to do them at "off hours" (very early or later in the day) when there's no/less divers on the site, you'll see more fishes.