These are not Montessori toys but do complement a Montessori setting.
Wooden train track
We actually got ours from Daiso and I bought a few pieces each time I wen to the store to build up the set. I think many of these sets are interchangeable but some more expensive sets will only work with their own pieces. I think it's a great toy to build on as you can ask family/friends to get the add on buildings, trains, people etc. Ours gets used in conjunction with the wooden blocks too.
Kids can get so much out of play silks, they can be used for no end of things. From impromptu dress up outfits to secret dens to flags. Etsy is also a good place to look for play silks, get a pack of coloured ones or some plain ones and dye them yourself, even make it a project with the kids.
This is another gift that can be added too, we got a basic set and then added to it. Hours of fun! And it has been known to have this set up with the train track (and absolutely no way to make it to the sofa!) The wooden sets like these are trickier than the plastic ones that slot together so I think these are suitable for a slightly older child. Ebi-kun got very frustrated when he first had his because anything slightly out of sync meant the ball doesn't roll properly. Now he has it down and he can work out what is wrong these days.
Absolutely every child should have a set of play food. We had a mix of wooden with velcro on them so you could chop them, plastic items (good when water gets involved with the playing) and felt items that I made. Etsy again is another great place to look for play food and Ikea have some great cheap options too.
Another fun but noisy choice! If you are a musical family then a selection of instruments are a no brainer, for us not-so-musical families a set of percussion can still be a lot of fun for impromptu dance parties and the such like. Ikea have just introduced a great musical instruments set too.