Trip to Ontario/Quebec

What time of year are they coming? That might impact my suggestions.
You already have given them a basic itinerary. Start in Toronto. They don't need a car at the start, can take the train from the airport into downtown Toronto. Pick up the car on leaving and go to Niagara Falls. Personally I would only visit the falls and then stay in Niagara on the Lake. If they enjoy wine they could try to add in a couple of winery tours there. Queenston Heights is also worth a look.
Niagara to Ottawa is a long day of driving ( despite the fact Google says it is 5 1/2 hours, you can be stuck in a lot of traffic.) If they wanted a break I have heard that Prince Edward County is very nice, but have not been myself.
I'm not familiar enough myself with Ottawa and Montreal to give all the must see sights. Again, what I woudl recommend will vary on if this is a winter or summer trip

In Toronto: the CN Tower used to be very popular with tourists (going to the top or eating at the revolving restaurant). The ROM (Royal Ontario Museum) might interest them. Evergreen Brickworks is a nice outing, and there is a free shuttle bus from Chester Station, or one can park there. https://www.evergreen.ca/evergreen-brick-works/ The Distillery District has some nice restaurants and shops. So does Little India. I'd suggest Casa Loma to most people, but people from Austria might be underwhelmed by it. The beaches are a nice place to walk around--we usually go to Woodbine Beach, as our daughter lives near there. Or they could take a ferry to Toronto Island. Harbourfront can be an interesting area, too.

Niagara Falls: one of my least favourite places, but, still, the falls are a must-see. Granted, for a tourist from Europe, the ticky-tacky, carnival atmosphere of the place, especially Clifton Hill, might be fascinating. They might enjoy Bird Kingdom. I did, when I accompanied my husband who was attending a conference this past June. We stayed at a nice B and B that I would recommend: https://www.booking.com/hotel/ca/a-night-to-remember-b-b.html There's a light-show on the falls at night, although it was cancelled due to smoke the night we were going to go.

Ottawa: My son and his wife live there, and we were there this past June, also (and have been twice before). Of course, the Parliament Buildings are good to see. I think there might be some renos happening there right now, though. Besides the usual Centre Block and Peace Tower, the East Block has a tour in which one can view some historical offices that are recreated there (such as John A. MacDonald's office). There used to be light shows on the Parliament Buildings in summer--not sure if that's still the case. The Museum of History, across the river in Hull (but an easy walk, bike, bus or drive) is wonderful. There are a lot of Indigenous exhibits there, which usually interest tourists from Central Europe. Speaking of Indigenous experiences, I've heard that Victoria Island is closed (what I used to recommend), but this one is not far from Ottawa: https://madahoki.ca/ Of course, the National Gallery is worth seeing--we never miss it when we're there. There are other worthwhile museums, too, such as the Museum of Nature and the War Museum. Not far from Ottawa, across the river in Quebec, are the Gatineau Hills, which are lovely, and they might even spot a bear or some deer there. William Lyon McKenzie King's estate is there and can be visited. There are some nice small towns with cute shops and good restaurants in the area, too. One thing we did in June that we hadn't seen before was visit The Diefenbunker, and I found that interesting, as I remember the Cold War from my childhood.
Edited to add: walking down Sparks Street (a pedestrian street), visiting The Byward Market (cool shops--they can have a "beaver tail" there (sugary, fried dough snack)), seeing the Chateau Laurier, and visiting the Musée de Monnaie (museum at the mint) are also fun/interesting activities.

Montreal: It's been a number of years since I was there, but the old town area is always popular. The Notre Dame Basilica there is truly beautiful. Of course, they should try poutine while they are there. When we were there, a local friend took us to "Frites Alors" which, she thought, had the best poutine. They could also tour the biodome on the site of the 1976 Olympics. And if they like amusement parks, La Ronde is a good one, if they are there in spring, summer, or fall.

My visitors are coming end of May first week of June. Thanks for your ideas. I will send along all the suggestions to them.
Lyn

In Montreal, there's the debate between two bagel rivalries which one is the best. I think it's between Fairmount vs. St. Viateur. My cousins who live in Montreal said I have to try those two fresh out of the oven and see which one I like. We did. Husband and I tried it and, oops, sorry to say we could not distinguish the two AT ALL. And, while we think they were good bagels, we didn't think they were THAT unusual or anything special about them. Sshhh. Don't tell folks in Montreal that we feel that way about their beloved bagels ;-)

Anyhow, your friends might enjoy going to each of those two bagelery and trying for fun.

Oh, and as for poutine: well, it's fine. We also didn't find them to be THAT special! Nothing against Montreal or Canada. Part of my natal family are Canadians :)

The Parliament buildings are closed for major renovations and will be closed for quite some time, so no tours of Centre Block, Peace Tower. More information here: https://lop.parl.ca/sites/Visit/default/en_CA

Your visitors will just miss the Tulip Festival, which is too bad, this year the tulips were quite spectacular.

Thanks for the info on Ottawa.
Any suggestions for places to stay in Montreal and/or Quebec city would be appreciated
Thanks
Lyn

In June, we stayed at one of the Sonder apartment hotels in Ottawa, and we liked it a lot. https://www.sonder.com/en-ca/destinations/ottawa/search?sleeps=1&neighborhood=all_neighborhoods&bedroom_count=0&bed_count=1&bathroom_count=1

We stayed at a private rental in Quebec City last time we were there, and it's no longer available, so I have no suggestions for there.

We just went to Toronto and Montreal for the first time last month. It was a super quick 4 day trip but these were our favorite sites:
Toronto: High Park, walking path along Lake Ontario, Kensington Market, campus of Univ of Toronto, Distillery District, Gay Village; Stayed at Park Hyatt.
Montreal: waterfront dock area on St Lawrence River, McGill Univ campus, Old Town, Mount Royal Park, St Denis Street, Bonsecours Market; Stayed at Hyatt Centric.

Our recent-ish TR below, which might prove useful.
I am done. the Torontonian in Montreal
https://www.fodors.com/community/canada/en-pays-quebecois-a-montreal-mt-tremblant-tr-avec-fotos-1715084/