Wife, myself, and 3 kids (12, 11, & 9) are going to Montreal for 5 nights. Arriving in the evening on a Sunday and staying until Friday checkout. Staying in Old City. Looking for recommendations for things to do while there. Doing some research myself now but I don’t have a lot to work with yet.

Here are some family friendly ideas - The Old Port has lots to do - pedal carts, boat rides, labyrinth, etc.
The Biodome at the Olympic park is an indoor zoo, close by is the Insectarium.
Perhaps the most fun activity is bicycling - there are many miles of safe bike trails throughout the city, and bikes are available to rent everywhere. The Lachine Canal trail is fun, with a lunch stop at the Atwater market.
There is a large amusement park - La Ronde.
This time of year, there is always some kind of street fest going on somewhere in the city.
If you can get tickets, Cirque Du Soliel is a mind blowing experience for all ages.
If you have a car, there is a very large zoo in Granby which s an hour east of the city. There is also Parc Safari, about a half an hour south of the city.

thanks zootsi. we did buy cirque du soleil tickets for the fireworks night. so we're excited about that. great suggestions!

Okay. These are some places I know and heard of. it's been some time since I've been to Montreal. so hope this helps. You can start by touring historic locations like Château Ramezay and the Notre Dame Basilica. Wander casually through Quartier Petit Champlain's quaint streets and look at neighborhood stores. Try to Visit the Montreal Science Centre, if you are interested. The Old Port shoreline, where you can rent bikes or enjoy a riverboat, is worth visiting. For some outdoor entertainment, visit Parc Jean-Drapeau, which has bike trails, the Biosphere, and the amusement park La Ronde. Finally, sample poutine, bagels, and smoked meat sandwiches from Montreal's legendary cuisine scene.

Here is my trip report (link below) with photos from our 2019 trip to Montréal and Québec City. First seven chapters deal with Montréal. AURA is a light and music show that takes place in the evenings at Basilique Notre-Dame de Montréal. Highly recommended. Get tickets online before you go (in the other link below). We loved both cities.
https://travelswithmaitaitom.com/montreal-quebec-2019/
https://www.aurabasiliquemontreal.com/en