Dollar Trees aren't as common in Canada - Dollarama is the big player here. But they are overpriced (lots of $4 stuff), so I was happy when 2 DTs opened in our city. We don't get the same variety of things Americans get, and everything is $1.25 here, but still some good bargains.
-Cleaning supplies, especially Totally Awesome Cleaner and Amoray toilet cleaner. I am not a fan of most of their laundry detergent, (too watery, so you need twice as much) but use the blue one with oxy-clean for towels and other non-clothing items. They don't offer a non-scented variety, and most of my family has skin issues with perfumed and scented detergent.
-Stickers. Our grandson loves stickers, and they have new ones all the time.
-gift wrap, cards, napkins, etc.
-some food items, but limited here. No freezer items, and we rarely get the cereal deals the U.S. gets. Crackers, some cookies, rice, spices are usually good buys if they are name brand and full sized. Coffee selections are horrible - won't be buying those again! Thin pizza shells - we love those for quick home-made pizza night.
-bread, depending on the store (ones next to grocery stores are limited in what they can offer due to non-compete rules). One of our DTs offers Dempsters breads, rolls bagels, etc. This is a super buy - those often go for $2.50 each and more here.
-first aid items are also limited here. No cough syrups, antibiotic creams, etc. But usually decent band-aids, disinfectants, cold packs and those sorts of things. They carry benzalkonium chloride swabs, which are great for scrapes and cuts because they don't sting.