A wonderful "problem" to have indeed.
If you've got the time to take a few practice rides, take full advantage of it.
Practice everything you can think of. She needs to be able to get herself on and off without pulling you over. She should always get a confirmation from you before making any start to mount/dismount so that you can brace yourself and the bike properly. In a parking lot or on gravel, I will always back out of the space and get on stable ground before letting my wife mount up. It just takes one more ingredient out of the equation for a problem to develop.
Get some basic hand signals or taps on your thigh sorted so that you will know when a washroom, (or real emergency stop), is required. Enjoy the silence as much as possible, because if you both enjoy the trip it won't be too long before you start looking at intercoms. They do have their advantages.
Try to sort out your route as much as possible before you head off. It's little fun stopping every few kilometers to keep checking, and as useful as they may be, maps on your tankbag require you to keep nodding down to see. Not only does this take your eyes off the road, but your wife will think you are falling asleep and start wacking you on the helmet!
Remember the extra wieght will affect stopping distances and the change in your centre of gravity will affect leaning into corners. Keep cornering pretty gentle until you are both used to it. Some people like to lean into the corners, but if you are both leaning in different ways or you scare her, it is a recipe for disaster.
I have also been learning from hard experience that my endurance level is somewhat longer than my wife's. This can make for some tense travelling. Fortunately you are setting off on a bite sized excursion. Ideally you can settle in one place and make some shorter trips from a home base as time permits.
As for routes, you look ideally situated. Look at the route along the Grand River from Brantford down to around Dunville. I think it is #54 for most of the way. I've only travelled parts of it, but it's a nice variation on the predominantely flat southern Ontario terrain. Also the Niagara Parkway from Niagara Falls to Fort Erie is a nice ride along the water. On the U.S side there might be something similar worth exploring. We went through there last week and there seemed to be some construction going on that made it rather awkward though.
If you travel west of St Catherines between Hamilton, (along #81 I think), there are all the wineries to check out too. Some good restaurants in Jordan and probably elsewhere. Check out Inn on the Twenty in Jordan for food and accommodation if inclined. http://www.innonthetwenty.com/
Hope that helps a bit.