Chocolate does OK at this time of year.
I have seen numerous requests for flavored creamer, depending on where the FPO is the PX may not have a good variety of bath items.
If you send anything in a can either make sure it has a pull top or try to remember to include a can opener. The biggest thanks I ever got was for a stinking can opener.
If you send anything that could leak make sure you put it in a baggie. And definately use the USPS flat rate boxes if you can. The PO can provide a list of what you can't mail.