If you like Japanese maples there are hundreds of kinds of those including dwarf growing ones. So if you have seen one you have not seen them all. However any of these being out on a balcony through a New York winter could backfire some time due to it getting too cold for their roots.

A crab would be hardier but I don't know which of those you might find that would fit the space. Even the comparatively small number of dwarf shrubby selections tend to be sold perched on long trunks and looking like a hat rack in nursery sizes. Also any that are selections of Malus sargentii will have stinky flowers.

Really as with so many plant selection questions it gets down to what is to your taste. And not just what will take the environmental conditions you have.

Thanks, Embothrium. Well, our taste is definitely NOT for stinky flowers, so we'll avoid the M sargentii. :-) I do love the maples, and if we had a LOT more space, we'd have many of those. Size isn't too much of a concern, as I'm happy to prune away and keep the tree manageable. Also, it's possible we'll be buying a house in the next few years, so we could take the tree with us and either plant it in the ground or at least give it a much larger spot to grow in.

Lilacs would definitely stay smaller though. How are those as container plants?

Look for a dwarf lilac with an extended or recurring flowering because a common lilac is not going to be very suitable for your current situation.

Thanks! Will do.