I had to Google tray favors to find out what they are (it's not a term we use in the UK as far as I know) and the first hit that came up was somebody's board on Pinterest with lots of little favours for all kinds of seasons - you might find some ideas there. Here's the
link.
I would go along with Bugga's suggestion of bookmarks - quick to make and useful for most people. How about little notebooks with decorated covers and maybe a pencil or pen?
What do the residents do with their days? Could you get any clues from that as to what might be good things to make them smile?