NB and PEI in a week?

There tidal bore viewing spots in Moncton right along main roads. These would not require a lot of walking. Check out what time it is schedule to occur and then arrive at least 30 minutes early-times can vary somewhat. Be prepared-sometimes it's very small but still interesting to watch the tide come in that way. While the viewing at Hopewell Rocks doesn't show the actual wave, it is mind-boggling to watch how swiftly the water level rises. You will be pressed for time to do much in either NB or PEI if you choose both as I am assuming you will take two days to get there. That said, I would take the bridge both ways. Definitely go to Charlottetown as long as your FIL isn't completely immobile. It's a great place to just wander around. You might want to consider renting a wheelchair (if he doesn't have one) to increase your ability to visit.

It's been a few years since we were at Hopewell Rocks in NB, but I know we went early in the day to see low tides, parked in the Visitor parking lot and had lunch, etc. and waited for high tide to come in. It really is spectacular to see both high and low tides. We were able to walk around the rocks when the tide was out, but I am fairly certain the visitor center has a large viewing area where you can view the tides. It would work for you in-laws. Parking a large rig should not be a problem.

A hint - if going to PEI, take the ferry from NB over to PEI and come back on the bridge. The ferry charges by the foot whereas the bridge is a set fee for RV's.

Sorry I can't help with the tidal bore - another tourist attraction but not in the same area as Hopewell Rocks.

On PEI, be sure to spend some time in Charlottetown and visit Founder's Hall. Also the play, Anne of Green Gables is enjoyable for all ages. It runs until Sept. 23rd.