Hey Cirrus, a big re-welcome back to Canada!
If you're interested in transit stuff, you cannot miss Exporail. Not on the island of MTL but on the South Shore, about a half-hour from downtown in St. Constant. Oooh the wonders! Locomotives, rail cars, trams.. about 100-150 of them. You won't be disappointed and you will need a full day. Weirdly, I doubt that many people on SSP - even the transit geeks - have ever heard of the place let alone visited. Regardless, this should be your main destination during your Montreal segment. If you don't love it, send me your receipt and I will refund your money!
http://www.exporail.org/en/welcome-to-exporail/
Otherwise, there are the usual many things going on in MTL, even in March. If you're here before 11 March, there's Montreal en Lumiere 23 Feb to 11 Mar. Food, music, art.. a little for everyone. Just go down to Place des Festivals, there's a slide (cool), ferris wheel (great views), zip line (couldn't tell you) food, drinks, music etc. It culminates in Nuit Blanche on the 11th.
http://www.montrealenlumiere.com/en Lotsa fun.
If not, I am loathe to recommend this but, if you're at all mesmerized by the fabled 'underground city - and you shouldn't be - this does make it a little more interesting than the series of metro access points and malls that it is. And there are some bits of the RESO, as it known, that are interesting design wise.
https://m.youtube.com/watch?v=yFteqEPQwDg
You might want to take the metro out to Olympic Stadium, a beautiful yet tragic structure, it is surrounded by the Planetarium, Biodome, Insectarium, and Botanical Gardens. There is this fabulous event - Butterflies Go Free - on at the Gardens which is surprisingly quite compelling. 1000's of butterflies flitting about, landing upon you, saying 'hey' (in butterfly, of course). I'm not much of a nature guy but this is awesome.
http://espacepourlavie.ca/en?m=0
Don't know how much of the Plateau you've explored but it is a many faceted beast. An adjacent hood that I love is Outremont. Bernard is a sublime street and Laurier too. As is Parc, the border between the Plateau and Outremont.
Finally, if you're here on 17 March you MUST attend the St.Patrick's day parade! Oh, I know what you're thinking "parade, whatever, boring" but no: this is possibly the funnest, most genuine, mass get together of Montrealers ever. Guaranteed blast!
If you want to go to any museums or attractions, might I suggest a Museum Pass. You get 3 days unlimited transit and free access to 41 museums. 80$ and it pays for itself, even if you only go out to Olympic Stadium. But you should also check out at least one of the 3 history of Montreal museums.
http://museesmontreal.org/en/passes/the-passes
I'm sure there's stuff that I've missed but I hope that I have been of some help.
Feel free to PM me when you arrive if you require advice and it would be a great pleasure for me and my gf to show you around - work and all that crap permitting.
Think EXPORAIL!