Canada

Good choices so far! A few other ideas:

-Museum of Science and Tech is an obvious kid-favourite.

-One I don't see mentioned as often is the Canada Agriculture and Food Museum at the Experimental Farm. Haven't actually been in the museum myself, but they do have live animals as it's a working farm, and I did see kids getting a kick out of that. It's also across the road from the Fletcher Wildlife Garden, which connects to Dow's Lake/The Arboretum if the kids have energy and patience enough for a little hike.

-Unless you really want to do a full river/canal boat tour (which admittedly is a good time), if you want to give the kids an 'on the river' experience you can take a water taxi from the locks to the Museum of History.

-For a less structured day/period, Mooney's Bay has what I believe is the biggest public playground in Canada. You can also toss the kids in the water if it's really hot (depending on water quality and comfort level). Keep an eye out as there may also be festivals going on depending on when you come (all pretty kid friendly as I remember them). Edit: Can also take them to see the Hog's Back falls while you're there, it's close by.

-House of Targ, like 2-3 blocks south of Lansdowne, has all-ages family freeplay on Saturdays and Sundays from noon to 8:00 PM. Bunch of pinball machines and arcade cabinets. Have seen elementary age kids having a grand time there when I've showed up early for a band/adults only freeplay.

-Depending on scheduling and available time, the Mayfair Theatre has a Saturday Morning Cartoons program, with cereal and people in PJs, once and a while. People and their kids seem to love it. Bookmark their website and watch for advance tickets, it sells out FAST.

-We have a baseball team - the Ottawa Titans [edit: NOT Giants]! They play at the old Lynx stadium on Conventry. If the kids like baseball/sitting in stands drinking soda and cheering on something they don't understand, they may enjoy it. Also have soccer and football at Lansdowne.

-If you want to get the kids into nature, Gatineau Park is a no-brainer.

-Vincent Massey Park is nice as well.

Might add some others as I think of them.

It may not be your thing, or your kids' thing, but the Imperial War Museum is really well done. Probably it could wait until they are a bit older.

I learned everything I ever needed to know about Baltimore from The Wire

I get that, though it does have some cool features (the Science Center is fantastic). But that trip was to see particular people rather than the city itself. We'll miss them, but we also don't feel super safe crossing the border right now.

Baltimore is certainly an unusual choice.