My first thought would be Apple Pie smell! I'd buy something to scent the office with that is associated with fall. You might also put together a grouping of small pumpkins (one for each staff member with a painted, smiling face is always fun) and lots of leaf cut outs of various colors, sizes, etc. You can plaster the cut outs all over the windows, etc. You can also make a scarecrow sitting on a hay bale, too. None of this costs much!