Question
I want to get a tree as a gift for someone with a greenhouse. It is located in New York State, but the greenhouse is kept warm  >50-60degrees year round- any ideas?

Answer
There are trees and there are TREES.... I am sure you are referring to something that will be tolerant of green house conditions, and not get to big to fit under the ceiling.
Here is a list of a few of the favorite trees, and large indoor plants that I can recommend.

Abutilon
Ficus
Rubber tree
Philodendron
Schefflera
Fatsia
Palms
Banana tree
Avocado
Citrus
Aralia
Deffenbachia
Norfork pine
Yucca
Corn Plant
Fiddleleaf Fig
Monkey puzzle
Dracaena
Most of these should be available in your local nursery or a store that sells indoor plants.

bakerplanter