Pines, larches, chinese junipers (not the stricta, never the stricta), rocky mountain junipers that are not(!) the columnar varieties like skyrocket, azaleas are cheap around here; 2 euros a piece for every cultivar but they're all small cuttings, small leaf ginkgo, hinoki cypress, and freaking wildtype sabina (not the tamariscifolia) is very hard to find in the US.
Those are what I'd love to find as small, medium and larger trees.
About yay high, any fruits that aren't grafted (anyone can cut those back whenever) and juniper cultivars like media, pfizer (because grafting new foliage is easy, and these are great cheap trunks).
If they have that kind of time, make them only pick the female junipers. The males will sell, but not as good as the females.
I know there's a huge wish for ume and mume, so flowering prunus are definetely good.
Also, make sure they check out accent plants in their free time. Could be a goldmine for them.