Dont send anything, dont give gifts.
Giving just a card makes you seem tactless, especially if they dont believe in christmas for example.
Don't give gifts either. Say you send chocolate to a tenant, one of the unit occupants gets it, eats a pieces, kicks off. You get sued, and insurance won't cover you, you potentially lose everything.
The risk simply isnt worth their gratitude, because thats about all you're getting.